Answer

Hello Rafi. Indeed, it is obvious that there is no coercion greater than coercion in matters of belief (as the Radbaz also wrote in his well-known responsum).
It seems to me that this is consistent with Maimonides, because he is speaking about people who deny out of their inclination, and in that case they are not coerced. But their children are already in the category of “captured children.” The assumption of Maimonides, as well as of the Sages, is that if a person studied and heard our tradition, he certainly understands, and it is impossible for him to deny it except deliberately. A person cannot innocently arrive at a mistaken conclusion unless he is a captured child who did not know and did not learn. In our generation it is clear that this is not true, and therefore my remarks apply only to our own generation. Today it is clear that even a person who has studied extensively can deny in all innocence, simply reaching the conclusion that there is no truth in it and abandoning it. Such a person, even if he knows the entire Talmud with the medieval and later authorities and the books of Jewish thought and Jewish law, is a captured child in the fullest sense.
I assume that in the past as well there were such types, but they were a minority. The presumption was that if someone denied, he was deliberate and not innocently mistaken (that is, coerced). Therefore the burden of proof was on the one who claimed that he denied in innocence. But today that presumption no longer exists.
And finally, you asked how it could be that a person truly believes and nevertheless denies deliberately. Perhaps this can be compared to someone who knows that adultery is forbidden and nevertheless commits adultery. True, denial concerns beliefs and not actions, and still the human being is a complex creature. He can deliberately put himself into a state in which he convinces himself that he is a denier. This reminds me of a remark I once saw in the name of Rabbi Shach (I think in a book published by his grandson on the Torah portion of Noah) that Cain was deceiving the Almighty. And he explained it with a parable: a person walks past the wires of the eruv and his hat flies outside. He looks right and left and sees that no one is there, so he steps outside, quickly takes the hat, and returns to the private domain. The Holy One, blessed be He, sees him, and yet he still feels that he is deceiving Him. It is clear to him that he is a transgressor and that the Holy One, blessed be He, sees him, and still he does it. At that moment he may even have built for himself a theory that he is in the right and is only trying to evade people who are mistaken and think this is a transgression, and still such a person is considered deliberate and not coerced. There may be room to connect this to the topic of someone who places himself into a state of coercion, but this is not the place, because the matter is lengthy.