Further investigation of the allegations against mastodon.art yields:
- No receipts, evidence, or concrete details, not in any subthreads or side threads either that I can see
- Lots of evasive answers about exactly what is being alleged
- Two people agreeing that mastodon.art is in the wrong
- One of which signed up to the UfoI (the no-defederation pact)
- The other of which operates an instance that's well-known for originating harassment

So, uh, yeah, I think I'm gonna call this one 'case closed'.