WIY: It’s not. The Rambam would call that Apikorsus as well (I think he says that B’feirush somewhere; I have to find where).
It’s fine to say that we don’t entirely hold like the Rambam on this issue, certainly because mainstream Shittos are far more mystical and less rationalist than the Rambam was. But trying to answer it up and say why it works within the Rambam is, frankly, an insult to the Rambam.