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.
It can definitely backfire. But it's impossible to know what online randos will respond to (people change their minds in different ways) and some folks do respond to having their bullshit called what it is.
Also important to remember the lurkers, who vastly outnumber us humble posters, and who are not going to be as turned off by hostility as the person on the receiving end of it -- especially when right next to the hostility you have someone making a more conventional argument.
I think the actually more salient issue is that, we're openly having fun clowning on these morons. Lurkers see us having an absolute ball and opt to join in. Recall that you can point to the origin of this online subcurrent as a mishmash of SomethingAwful and largely apolitical 'weird twitter'.
Great point -- people, they love fun!