It's the identity warriors. Think BLM, Tumblr, the radical identity politics people. We don't call them alt left, we just call them Black Lives Matter, Tumblr whateverisms. They serve the same purpose of being an echo chamber and highly ideological.
To a certain extent, this. The alt-right is pretty irrelevant - its main base of support seems to be on reddit and on a few fringey far-right websites; the few 'representatives' that it has in the real world are largely attention-seekers like Milo Yiannopoulos (whose shtick is basically a watered down and less amusing version of that of Taki Theodoracopulos). It's only real relevance is, as has been mentioned, a big bad bogeyman used by liberal 'rightists', standard liberals and left-wingers to browbeat the mainstream right with whenever it attempts to touch certain issues.
On the other hand, what Potus described as the alt-left has clearly moved well beyond its Tumblr stage and is busy infiltrating (or already has) universities, online publications and indeed political parties, and is busy moulding the minds of a generation of impressionable young people. The alt-right, I guess, is a reaction to this, but it's an incredibly weak and counterproductive one, and, like I said, is largely confined to the fringes, unlike the 'alt-left' (which is part of the reason why it isn't called the alt-left).
