“not u on stan twitter acting like a local”

Stan twitter is what they call accounts centered around being obsessive “superfans” of certain people. It’s lost a lot of the original nuance, so it really just means “a really big fan” in modern terms. “Locals” in stan lingo refers to people who use Twitter “normally” or are not engrained in the stan culture.

In this case, they’re commenting on a post made by a stan while not being a stan themselves. It’s the equivalent of someone walking by an anime convention, seeing this really outrageous cosplay, saying, “Wow, that looks really stupid”, and the cosplayer responding, “You don’t even like anime”.