I see this a lot and I really don't understand it. Here I am on lemmy.world, replying to you on sh.itjust.works, on a post from lemmy.ml. Clearly neither of us are browsing just our local instance, and indeed I imagine most people are not. So what does it matter what instance anyone is on, beyond instance-specific defederations?
I won't link it because it's a reddit link. But there's damning footage of large pixel circles of white or random colors spreading at the same instant, in a way that wouldn't be possible by regular users. And these pixels don't have an attached username, it's like the canvas has been totally reset. Bots would show a new username.