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.
Theres definitely a lot of smug arrogance from libs. I think the tendency to be antogonistic towards a leftist perspective or point comes from a cognitive dissonance. If they're American, they've likely been indoctrinated by institutions like the media and the education system. Anything that deviates from that or refutes it with facts and or historical context makes them question what they know while the propaganda makes them feel they're in the right.