Liability is not binary. There is a qualitative change in risk as you transition from “I subscribed to 100 actively moderated communities that I read and am familiar with” toward “I subscribed to everything there is including the worst of the worst and I didn’t realize I was doing so and don’t look at the results”.
Also, moderation activities federate. So even if a rogue poster does “contaminate” the actively moderated communities on a well-admin’ed instance… when those mods and admins delete the offending material they’ll automatically cleanup your instance as well. As a result, it’s the creepy crawly communities that don’t clean up or don’t want to clean up that generate the lion’s share of risk.
Is it 100% safe to sub to well-moderated communities, no. You have to know your local laws and protect yourself. Do you do yourself favors by running lemmony? Also no. These two statements can be simultaneously true.