Gruelly is pointing out that the phrase rose to prominence from the right half of the party asking the left to fall in line for the general. In this instance the left is asking for the right to fall in line for the general, and the right is having a meltdown about it.
That's not actually true, though. I was there in 2008 when the phrase rose to prominence and it was mainly being said by progressives trying to convince moderates/centrists to vote for Hillary. I don't know if or when the "right half of the party" ever adopted it themselves.
2008? I hadn't seen it that early, I saw it rise to prominence in 2016. I think a lot of people may have also heard it then, when it was often coming from the right half of the party in my personal experience.
Wait, I miscounted. 2016 was the one that was Hillary vs Trump, that's when the phrase started. Still I don't remember it being said by the right half of the party. I don't think the party even had a "right half" back then.
Hillary vs. Bernie was very much the right vs. the left halves of the party! I'm using shorthand because of character limits, but you can frame it in a bunch of ways - corporatist vs not, health insurance vs health care, PATRIOT act good vs bad, Iraq war good vs bad (at its start) etc.
Maybe I was still a little young during that time and wasn't paying enough attention (the election before that was the first time I was old enough to vote), but I don't remember anyone on the left being on the right side of any of those issues, not even Hillary.
Anyway, I still remember 'vote blue no matter who' mostly coming from people who voted for Bernie in the primary, basically saying, "I'm as disappointed as you are but don't forget what's at stake if Trump (and the rest of the Republicans) wins this."
The accusation is that a significant chunk of the right half of the party was using the phrase disingenuously, which I think is probably correct, much to the irritation of those of us on either side who actually meant it.