It's not outside the realm of possibility, they have an IP address and an e-mail address and sometimes that can be plenty to go on. Very illegal to do that though, from a privacy perspective, but I don't think that bothers most Lemmy admins, since the overwhelming majority don't even think they need to have a Privacy Policy.

That said, if your e-mail address can't be tied back to real you, then they don't have much. Even your real IP address is not that revealing of your true identity. Most people actually dox themselves bit by bit. They'll give away what they do in one thread, where they are in another, etc.