How likely is it that this thread's closure was really inadvertent?

Given the one click process Joe Offer described, I'd say it's very likely. I don't know the mechanism here but it is also possible that the "culprit" was unaware of what they did.

Is it really impossible to establish whether this thread closure was intentional?

My best guess is that in the absence of a trace mechanism (which incidentaly I do not have on the sites I am involved with) is that the server logs would contain an entry giving some clue. But even if that record existed a) I would doubt Joe Offer would have access to it and b) I can't reasonably see anyone searching through those records to attempt to find an answer to such (as a one off - one might be concerned if it was happening daily) trivia. I can assure you that at the Annexe which will have considerably smaller logs than here, there is no way I'd searching to attempt to resolve your complaint which IMO is blown up beyond any imaginable sense of proportion.