Clearly the actions constitute a grave sin against the faith and an unspeakable scandal.
Yet actions cannot render one guilty of "heresy"; actions are not beliefs. They can render one "suspect of heresy". So, for instance, if before Vatican II you saw someone going regularly to a Protestant church for services, they become suspect of heresy. Then the Canon Law presumed someone guilty of heresy after a certain period of time during which the suspect behavior continues. If, however, there's evidence to the contrary, while the actions would constitute a grave sin against the faith, it may not actually mean being a heretic. So, for instance, someone might just be going to the Protestant chapel because he has a friend who goes there, and perhaps even with the intention of going to convert some of them, not knowing that it would be forbidden even for noble motives.
So the truth is somewhere in between. Yes, the actions can be judged as reprehensible, scandalous, and a grave sin against the faith. No, the actions do not by themselves prove heresy. I doubt that Francis is actually a secret Muslim. He most likely believes that he's fostering some kind of "good will".
Again, the core heresy here is religious indifferentism and the rejection of EENS ... not that he's a Muslim.