If I'm not mistaken, to be a Noahide you must accept that the Torah is unchanged and from G-d. (I am not arguing that this is one of the 7 Noahide Laws by the way, I'm merely saying that this could be a prerequisite to be considered a Noahide).
However, according to my understanding, Muslims are not Noahides, since they reject the infallibility of the Torah. In other words, they say that the contemporary Torah has been altered from the one that Moshe Rebbeinu received at Sinai. Therefore, according to their beliefs, they would not feel compelled to abide by the 7 Noahide Laws because they don't believe it to be from G-d in the first place.
I say the above in light of what Maimonides says in Mishneh Torah, Melachim uMilchamot, Chapter 8:
Anyone who accepts upon himself the fulfillment of these seven mitzvot and is precise in their observance is considered one of 'the pious among the gentiles' and will merit a share in the world to come.
This applies only when he accepts them and fulfills them because the Holy One, blessed be He, commanded them in the Torah and informed us through Moses, our teacher, that Noah's descendants had been commanded to fulfill them previously.
However, if he fulfils them out of intellectual conviction, he is not a resident alien, nor of 'the pious among the gentiles,' nor of their wise men.
So my question is: What categories do Muslims fit into in light of what Rambam says above? Are they Noahides? If not, what are they?