How do all these stringencies that apply to a descendant of Noah more than to a Jew fit with the Talmudic text:

“For Rabbi Yose son of Rabbi Hanina said: Every commandment that was stated to the descendants of Noah and repeated at Sinai was stated for both these and those. If it was stated to the descendants of Noah but not repeated at Sinai, it was said for Israel and not for the descendants of Noah… On the contrary, from the fact that it was not repeated at Sinai, it should be said for the descendants of Noah and not for Israel?! There is nothing that is permitted to Israel and forbidden to an idolater.”

What stringencies? What’s the question?

You could ask how a Jew is forbidden to a gentile woman, while a descendant of Noah is permitted with her. You surely understand that this does not contradict the Talmudic text you cited.

I’m asking because you can see specific details of Jewish law where a descendant of Noah is treated more stringently than a Jew is treated more leniently than him (Parashat Derakhim brings quite a few). I’m asking how that fits with the Talmud, which implies that such a situation cannot exist.

It seems to me I’m missing something here in the definition, but it’s hard for me to put my finger on it.

I still don’t understand. I gave you an example above, and it certainly doesn’t contradict it. Can you bring another example that you’re unsure about? By the way, Parashat Derakhim itself discusses this, and Rabbi Kook also talks about it in Pri Etz Hadar.

Even regarding this matter itself, of marrying two sisters. If it is forbidden to a descendant of Noah, then the answer that for a Jew it is permitted (for example because he converted them, and a convert is like a newborn and has no relatives) still doesn’t help in the end on the level of the “descendant of Noah” that exists within the Jew.

Rabbi, I think he is asking that the very position “that they left the category of descendants of Noah leniently” stands in contradiction to the Talmud, which says there is no such thing as a Jew being treated more leniently than a descendant of Noah.

So about that I already asked you why you don’t relate to a simpler case: a descendant of Noah is forbidden to marry a Jewish woman, while a Jew is permitted with her. It’s like the argument a fortiori regarding theft: just because it is forbidden for me to put my hand into your pocket, is it permitted for me to put my hand into my own pocket? All the more so, from the fact that you are permitted with my pocket, you should be permitted with my pocket. Or more simply: why is a Jew permitted with a Jewish woman? After all, within her there is also a minor descendant of Noah, and from the aspect of the descendant of Noah within her he should be forbidden to her.
This is empty pilpul.
Lulbon, my answer to your question is written above here. They left in a lenient direction, for example in that they are permitted with a Jewish woman or permitted regarding Sabbath observance, and that does not contradict the Talmud.