It’s at the end of his book Mitpachat Sefarim (where he argued that parts of the Zohar were written long after the period of the Tannaim). I don’t remember encountering anyone, in his generation or afterward, who treated his arguments on this seriously (as distinct from his claims about the Zohar).

Is it really not interesting to the Rabbi whether Maimonides wrote the Guide for the Perplexed? Not that it would affect whether one accepts the arguments, but it surprises me that it isn’t even a very interesting historical detail. (I find it very interesting. It’s a very significant biographical detail about a major figure, and it’s simply interesting.)

Agreed. It’s less interesting whether Maimonides wrote it. I’m interested in Rabbi Emden’s reasons for his claims.
I know it’s written in Mitpachat Sefarim. I’m interested in the content of the arguments. I don’t have access to the book.

Not very interesting.

It matters in cases where people want to discuss Maimonides’ view in the Guide for the Perplexed in comparison with the Mishneh Torah, and resolve contradictions.
If these are two different people, there’s no need to reconcile anything.
But as far as I know, in Maimonides scholarship there is a consensus that this is one author.

Both Mitpachat Sefarim and Birat Migdal Oz are on HebrewBooks.

There he makes general claims about the content (and in truth they are very strong claims), not specific and/or historical ones. Therefore he concludes that such a book does not suit Maimonides (“a mistaken student,” “changed circumstances of nature,” “a forger who wanted to hang himself and attached himself to a great tree,” etc.).

What bothers him is that the Guide reduces the Torah to unimpressive explanations. In Rabbi Emden’s view, the reasons for the commandments in the Guide are “words of mockery, absurdity, and ridicule.” The interpretation of the Account of Creation and the Chariot is built on “thin, meager, withered human speculations, such that even the one who says them would surely loathe them.” (In Migdal Oz, Even Bochan, he adds that when the Sages warned not to expound on the Account of Creation before two people, they certainly did not mean “to know the terms matter and form; and to know that all lower bodies are composed of the four elements—fire, air, water, and earth; and that the matter of the spheres is a fifth body beyond comprehension; and the supposed number of the spheres—there is no depth in these matters at all, and most of them are flimsy conjectures and guesses from gentile philosophers, whose wisdom is of no benefit to them; they confer no perfection in faith or in intellect, nor even sharpness.”) Nor is it possible that all the depth in prophecy and the like is exhausted by Maimonides’ explanations; to truly descend into that depth, Kabbalah is required, not following heretical philosophers, etc., for God set a limit to natural inquiry and stationed human intellect there.

He also has a famous claim that Maimonides rules that someone who keeps the seven Noahide commandments “by intellectual decision is not even among the wise of the nations of the world” (apparently a mistaken reading in Maimonides; it should read “but” instead of “and not”), and if so, how could he praise and be drawn after the sages of the nations of the world like Aristotle and others? He also hints that in several places in the Yad HaChazakah there are derogatory descriptions of the philosophizers.

Obviously, claims like these are far from sufficient to undermine such firmly established authorship of a book.

Did he need to inform us specifically “not very interesting,” meaning: but to a reasonable degree it is interesting? (Not for the sake of the book, but for the sake of knowing the man—out of curiosity about a respected and/or somehow close personality, not because of anything normative.)

Amiram, thanks.

He’s coming to tell us that that is more interesting than the identity of the author.

That’s not what we heard at all, because here I am before you—living proof—that I’m more interested in the content than in the identity of the author, and still I’m very interested in whether Maimonides wrote the Guide for the Perplexed. (Heaven forbid that I myself should wade through all the nitty-gritty and dig into every detail; but if there were some summary study saying there are strong challenges, I would certainly read it.) There are very many interesting things in the world, and not all of them have to do with content. Even politics in Israel and around the world interests me, even though its influence on me—and all the more so my influence on it—is essentially zero. History is moderately interesting (I don’t always know why, but that’s how it is), and biographies are also interesting even if they don’t provoke deeper thoughts. In short, I was surprised, and I interpret “not very interesting” to mean that there is at least some interest there—namely historical curiosity, especially toward people about whom one has some strange feeling of closeness.

There’s no need to take his claims seriously.
For anyone who engages in Kabbalah is engaging in the false Torah of charlatans who wrote a Torah out of their own imagination and the musings of their hearts, and is denying the Torah of Moses our teacher.

Posek, which claims of whom are you referring to?

Rabbi, can’t one extract from Rabbi Emden’s discussion that there is substantive authority? At most, what one can extract is that Rabbi Emden thought Maimonides had substantive authority…

And even then he found excuses not to accept it: forgery, etc.

A. The question whether there is substantive authority is something a person must decide for himself, because it is itself a matter of opinion (or understanding) and not of action. So adding Rabbi Emden to one camp or another on this issue makes no difference.
B. Rabbi Emden says about himself more than once that in halakhic ruling everything depends on the matter at hand and that he shows favoritism to no one, including specifically Maimonides (and I’m not expert enough to judge based on the content, but this is the kind of thing that can be checked, etc.). That means he certainly did not attribute substantive authority to Maimonides—in Jewish law, and all the more so in matters of opinion.
C. It is hard to believe of such a clear-eyed person as Rabbi Emden that he really thought the Guide was not by Maimonides. I think a decisive part of the point was to enable him to express his sharp criticism of the book’s content. After all, not everyone who says “a mistaken student wrote it” really thinks so; sometimes that’s lip service outwardly, and sometimes inwardly too. (According to this, all the more so he did not think there was substantive authority. How many times has Rabbi Michael Abraham mentioned the Magen Avraham on “how can they accept from him”?)
[D. By the way, I myself think they are both right. Maimonides is right that these are likely the reasons for the commandments, and Rabbi Emden is right that it’s absurdity and mockery. Regarding the Account of the Chariot, I don’t believe Maimonides hit the intent of the Sages, but it also doesn’t seem to me that their real intent was something more successful.]