You seem extremely confused about who that phrase “alt-left” is used to describe. Whereas “alt-right” was created by Richard Spencer, a white supremacist, to rebrand the usual white supremacist/Nazi/fascist subculture, a subculture that has generally embraced the title with great alacrity, “alt-left” (in its current form) was introduced by nut-right sites during the last presidential cycle then picked up by Clintonites as a smear of progressives. These progressives do not “embrace a Manichean identitarianism” as you describe. A foundational claim of the “alt-left” smear, in fact, is that they’re a bunch of white male sexists who are, at worst, racists and at best, try to downgrade the importance of issues affecting POC. Rightists, meanwhile, use it as a general-purpose smear and, as an ADL analyst noted, simply attach it to anything vaguely liberal that they don’t like. It doesn’t actually mean anything.
“In many ways AntiFa, BLM, Occupy and the more radical elements of progressivism really are a mirror image of all the various forms of white supremacy.”