Both the Onion and the Hard Times made pieces about how Buttigieg is ackshually a Republican LOL!

I don't get that as a joke or even a serious implication. The narrative constantly peddled out by this same wing of the Sanders base is that at best, the Democratic Party is the lesser of two evils, or is at worst a corporate, imperialist entity that's only better than the GOP on trite social issues.

Buttigieg was objectively on the more progressive end of the folks who ran. I think it's clear that the joke alludes to the fact that he's a white Midwestern veteran, probably with some crap about McKinsey mixed in too. Either way, the implication is that the Democratic Party is at odds with those things. So.