Answer

A. It is possible that Maimonides thought so. But that does not mean that this is actually the case. In my humble opinion, it is very likely that there are verses that were added later. As is well known, a number of errors entered the text of the Bible, and there are disputes about it (after all, there is even a Jewish law that deals with such disputes, which rules that we follow the majority of textual witnesses). So it is entirely possible that there were also disruptions in the transmission process (of course, even broader ones than the places where we know that a dispute or corruption occurred). Not to mention later editing that the biblical text underwent (the stitching together and editing of documents), and the like.

B. Maimonides also writes that there was never any dispute about a law given to Moses at Sinai, and it is clear that this is not true (see Havot Ya'ir, sec. 192, at length, and much more). In several places, Maimonides' way is to subordinate the facts (= what is found in reality) to theory (= what ought to be). Those who understand this know that this is the main point of Nachmanides' critiques on the roots Maimonides sets out: Nachmanides is an empiricist—sticking to the Talmudic facts—while Maimonides is a rationalist—subordinating his interpretation of the passages to reasoning, to theory.

C. By the way, if one wants to speak well of Maimonides and judge him favorably, there is definitely room for two other interpretations:
1. This is a normative statement, not a historical one. That is, his intention is to say that one should relate to the entire Bible as though it was all given at Sinai. That is its normative status, but this is not a historical claim. This is how one should understand the saying, "its general principles and particulars are from Sinai," and some say: its particulars were given in the Tent of Meeting (Rabbi Ishmael and Rabbi Akiva). Likewise, "everything that an experienced student will one day innovate" is a normative statement, not a historical one. Replacing facts with norm is itself a common norm. Thus Tosafot say about a law written in the Talmud that one is liable to death, that it is actually a rabbinic law, but they wrote that one is liable to death in order to show its severity.
2. Perhaps Maimonides did not really think this, but thought it important to say and present things this way for the sake of the masses. He himself does the same move in his commentary on the Mishnah to Rosh Hashanah regarding Rav Saadia Gaon, who said something puzzling, and Maimonides writes that Rav Saadia certainly wrote it only as a response to the heretics and did not really think so.
And in general, Maimonides definitely supported esoteric writing, as explained in his introduction to The Guide of the Perplexed.
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Questioner:
Thank you for the answer.
The problem is that this is not only Maimonides. Maimonides relies on the Talmud in Sanhedrin.
Is it said there that these are aggadic statements and therefore not binding?
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The Rabbi:
First of all—yes. After all, even in Jewish law itself (for example regarding killing a louse on the Sabbath), errors entered the Talmud. I will add here briefly that there is no point at all in speaking about concepts of authority with respect to facts. If a fact is not true, then one cannot demand that I accept it because so-and-so has authority about it. And even if they demand this of me, I cannot comply, because at the end of the day I think the fact is untrue. This is unlike laws/norms, where one can demand that I observe something even if it is not true.
But in the Talmud it is also easier to explain that this is a normative statement and not a historical one. It says that one who violates the words of the sages is liable to death, and that whoever recites havdalah over a cup will have male children, and that because of the sin of vows a person's children die. All of these are statements meant to push a value-laden outlook, not factual statements about reality.
I will elaborate more on all these matters in my books on theology (I am currently writing a comprehensive trilogy on updated Jewish theology: vol. 21—philosophy, vol. 22—Jewish thought, vol. 23—Jewish law and meta-halakhah).