“Islam is a syncretist cult.”
Which is why for centuries The Church held (some of us still do) that it was in fact a Christian heresy, like Arianism or Docetism. Like most heresies, it can be recognized by its fruit.
That was why Dante placed Mohammed in Hell as the great heresiarch/schismatic.
However, Mohammed was a tribal Arab and was probably from a pagan background, so I’m not sure you could call him a heretic (pace Dante). Don’t you actually have to be an orthodox something-or-another first in order to develop a heretical form of it?
That said, I believer there has always been some dispute over whether one should consider it a Jewish heresy or a Christian heresy (or maybe a pagan heresy, too, if the pagans had any doctrine). And Mohammed was actually pretty crafty in the design of his cult and much less “idealistic,” shall we say, than most heretics, who actually thought they had a principle they were defending. He did it for conquest, pure and simple. And the benefit of his tribe.