Earlier quoted context omitted.
> it's not even accepted colloquially in spoken English How the hell would you hear the apostrophe in "spoken English"?
That's exactly his point. You don't hear the apostrophe but you do hear the "s," meaning that Thomas and Thomas' cannot be distinguished. And so Thomas's must be used instead.
(My personal prediction: give it 100 to 200 years and we're going to drop the trailing 's' in all these cases. "Cat'" will just be pronounced "cats" and understood to mean "an adjective indicating the noun is owned by the cat").