But usage of that terminology is the best steelman I can see for Petruchio's post.

Fair enough. I also don't think this use of 'valid' is a good idea (how do we distinguish between sound mathematical arguments and, at the extreme, arguments that validly take contradictions for premises?). Also, what happened to dear old Tarski?

I guess the answer to my original question is 'yes, "tautology" is being used in a non-standard way'.