Great point!! This is the reason why I included this paragraph in the article we are discussing:

"It does not seem like too far a stretch to suggest that the Sufis walked a very thin line between extreme observance of Islam and outright heresy. Instead of becoming outliers, those who focused on the esoteric, spiritual aspects of life did so within the context of Islam, because there was no other way other than to risk life and limb as a secret society. "

I agree with you about the Abrahamic religions being mired in separation, but in taking a look at several key historical Sufis they seem to have found something that was beyond the teachings of these religions. I suspect that they remained a part of Islam only because to do otherwise would be to risk their lives from reprisal. No doubt the Sufis I mentioned understood, or realized, that there is no separation, so perhaps their religious affiliation was nothing more than a way to survive in a hostile and oppressive world of Medieval ideas, religion, and practices.