Anarchists, socialists, and antifascists have been traditionally grouped under the umbrella of "the Left". It's odd seeing people try to push parts of the original Left out into the "Alt-Left", almost like gentrifying a political space.

(Antifascism itself dates back to early 20th century opposition action against fascist tides in Italy, Germany, and Spain; the comparison to the modern phenomenon of the Alt-Right feels historically inappropriate.)

> Is Antifa mainstream Left then?

It substantially overlaps with the mainstream of the left; note that in the US, the dominant (though decreasingly so) faction of the Democratic Party is center-right, not left at all.

What exactly is your question? You have about 3 layers of circular reasoning and undefined terms.