> Goomba is assuming contradictions are coming from the same person, presumably b/c it's coming to the Goomba through the same app.

Its because it comes from the same political faction. In general people are open about A when A seems palatable, and openly B when B seems palatable, but they almost never admit to do that when its obviously wrong to do so.

That is the rational part of the fallacy, even if these are different sets of people you can still tell they are biased since they never appear in the threads where its obvious they are in the wrong.

For example, lets say in a thread where a white cop shoots a black guy you find a lot of republicans say "this is just statistics, nothing to see here". Then in another thread where a black cop shoots a white guy republicans pour in and argue this must be racism and we should investigate! Maybe it isn't the same set of people, but its still a strong sign of problematic bias that they only choose to speak up in those particular threads and not the others.

Every political side everywhere does this, and that is why people started calling that out.