LU: I agree with you (about non-Jews’ olam haba), and thanks for mentioning the rabbi’s opinion. I think this is one of those things that is not set in stone, and a range of opinions is possible.
Rambam seems to suggest that someone who doesn’t believe in Hashem will have no afterlife whatsoever. But Tehillim says each person is rewarded according to his deeds, which is the principle on which our Olam Haba is based, and it only makes sense that a just and merciful G-d would do the same for non-Jews.
In addition, I don’t know Rambam’s source for his opinion, but it’s important to point out that for many of the statements in our tradition in which a tanna says “Anyone who does X has no share in Olam Haba,” it is impossible to take it literally, because in that case hardly anybody would have a share. (Who hasn’t embarrassed someone in public at some point, even accidentally?)
Of course, it is also possible that there are additional complexities that we don’t understand (for example, perhaps a non-Jew who violates one of the Seven Mitzvos will have to have another gilgul in order to fulfill it before obtaining an eternal afterlife during which they will be rewarded according to their deeds.)