“That is true, unless one considers Islam to be a (quasi-)Christian heresy.”
Well, I have no doubt at all that Mohammedanism is a sort of Christian (really Nestorian) heresy. I suspect I am missing your point. That Mohammedanism is a Christian heresy doesn’t have much if anything to do with the religious and secular phronemai of Eastern Christians in my experience.
I’m not seriously suggesting it, necessarily, but one could argue that your heretics merely got more out of hand. :-)
This is in regard to your pointing out that Orthodox are more influential among Eastern Christians.
It was, perhaps, a superfluous point.
BK, Belloc certainly viewed it that way in the 20th century, and if you take a close look you can see why.