Every liberal does it too, from center right radlibs to far-right "conservatives": the most extreme right fringe liberals hate the mainstream liberals for not being bigoted enough, the mainstream libs hate the radlibs for not being cruel enough, and the radlibs hate the left for not being chauvinist enough.
Denouncing chauvinism in particular is like a liberal moral event horizon, a cardinal sin against their self-interested belief in the righteousness of the imperial hegemon that keeps the treats flowing at gunpoint.
I think it is more "They hate the possibility that they could be wrong about things." including their morals and politics.
A lot of liberal politics is comparing themselves to fascists and saying, "hey at least I'm better than that guy". They can define themselves by being the only ideological position that tolerated minorities and is "moral". Then leftists put an end to that idea because we are even more relentless in our support for marginalized groups. It's a big reason why liberals love to pretend many socialists and communists are huge bigots, it's why the proverbial Bernie bro became a boogeyman. If liberals don't have their moral high ground, they don't have anything.
Oh damn, I guess that's the real horseshoe theory. Or I suppose the reasoning behind it. If "tankies" are the same as fascists, there is no cognitive dissonance for a liberal, because they are still morally better.