No, you couldn't be sued for saying Alice was flat or Mario-like.
Yes, you could be sued for saying Liza was a drug addict.
Yes, you could be sued for saying Michael Jackson was child molester.
No, no one was ever "legally able" to make those claims (whatever that phrase means).
Why didn't Liza or Michael sue? No one knows except for Liza, Michael and their lawyers. Maybe the rumors were true, maybe they were false. Michael settled out of court when he was sued, but settlements are not public record.
Next question?
FYI, this case is gathering interest not because people are really interested in who Marty Thomas slept with, but because Marty Thomas is a Davey taking on a huge corporate Goliath. But more than that, "Anonymous Internet Slander" can easily be portrayed as "Cowardly Anonymous Cyber-Bullying."
This case has legs.