I think he just independently arrived at it on his own, with this very specific definition as applied to this one particular establishment segment of the Democratic Party, because he had really strong feelings created because all his pro-Democrats-winning-elections productive feedback was being short sightedly ignored. He's just trying to get through to them how important it is for them to start using more successful tactics to win the election and defeat Trump, because he totally thinks that's important and he's working hard to try to make it happen.

But then, independently, a whole different group of perfectly legitimate leftists invented the same term and started applying it (in front of a different audience with a somewhat different receptiveness level to transparent bullshit and groupthink), but using a different meaning and framing, which they also independently came up with, to encourage people to the totally different but still organically arrived at result of not wanting to vote for Joe Biden.

Or, wait, I'm not sure I think that. Now that I type it out it seems a little farfetched all put together. There must be some explanation, though.