Unless the users themselves are directly posting links to the content and hosting it (the pirated content) on lemmy, no instance would be held liable. Anyone defending this has no idea what the fuck they are talking about.
That's a bit naive, knowing what we know about the sharks that run the large media corporations. For your average instance owner, it's not a question of being found not liable, it's the fact that you as an ordinary guy with an ordinary life and an ordinary income suddenly have to defend yourself legally with all the exposure and expense that entails, from day one.
I don't know about you but engaging a lawyer and going to court to defend myself would be a massive financial drain. And to risk that on simply the hope that a court might find in my favour is far too big of a risk. Then add on all the unwanted public exposure, the internet notoriety etc. Fuck that.