They would be sued for not taking down the post. If you post something here that is libel then someone can sue to force Paul Graham to take it down.
If MMM gets a lawyer he's going to end up defending the content of the post, not his right to operate his site independently.
Also, the actual quote was "This scam product has been discussed here before. Stay away. It's a marketing ploy." Which I guess may be legally actionable, but given their behavior seems like it's quite likely true.