In order to win a defamation claim a person must prove serious harm, and that would mean having a reputation to cause harm to. If someone, hypothetically, had a business instead of several dissolved businesses, or someone was a solicitor or barrister registered with a professional body and practicing reserved activities, then hypothetically you could possibly claim harm was caused to one’s reputation. That’s if the defendant could not prove it was both factual and in the public interest, and, hypothetically, the claimant was themselves not behaving in a way they previously were before the courts for. This is just to give you an idea of how things could work. Obviously, the intention may be to cause financial hardships to a person, hypothetically, and in such a case that would only be for someone who engaged the services of an actual lawyer. I, with no legal training, successfully won a claim myself against a previous LL.
I think, although time consuming, based on SH behaviour and harassment of MN and accusations of radicalising young people ( actually @ them repeatedly on twitter ) and intimidation tactics online, it would be quite achievable to fight any vexatious harrasment claim, since the poster on MN never directed anything at SH, was not sending messages to SH or making up lies etc etc the whole thing seems really rather pointless. Nothing can come of it really... possibly finding out the details of the MN member but surely they wouldn’t want to do so to harrass that person? Or add them to a database? Surely not.