The alt-left is culturally libertarian and fiscally hard left, and rejects 'identity politics' which they see as alienating whites. This is abundantly clear on social media where they make their feelings fully known. It's not just about Russian bots, because many actual real life Sanders supporters and media outlets (Intercept,TYT,Chapo Trap House) parrot the same narratives. Many of them were against the Mueller investigation and the impeachment, and are sympathetic to Trumpism, dislike Democrats, etc. Basically the alt-left and alt-right are two sides of the same Putin backed coin.
If Sanders really cared about expanding his movement he would've dropped out and endorsed Biden, to build good will, get people united.
But this latest stunt furthered by his supporters (and Sanders has not come out to denounce it) is why regular Dem voters will never like the Bernie Bros/alt-left because of their toxic tactics - which are indeed unique to them - despite their excuses.
primary today, I would vote for: Joe Biden