Democrats don't go out of their way to disavow the more extreme elements of their support either. There were plenty of fairly horrible things said by various BLM supporters, and some pretty extreme things said by members of other parameters of the Democrat coalition. And Dems were mostly silent about that because they didn't want to antagonize potential voters or risk losing their progressive street cred.
The "you must disavow everything offensive said by anyone who supports you" standard was only applied to one side in this election.
Eh, but was the far left, e.g. Antifa, rabidly pro-Bernie the same way the alt-right was rabidly pro-Trump? I think Trump's candidacy was pretty unique in how much it appealed to the most extreme members of his party, relative to all serious candidates in both parties in recent years.