The proof is in the pudding. They may not be happy there, but do they work? And what if the tube only last ten years instead of 20? AFter all, 6V6s should'nt be happy in just about anything Leo Fender ever stuck them in.
If the issue is plate dissipation, yes the 12AT7 can handle 2.5 watts and the 12AX7 only 1.2 by the book. But just how much dissipation does the reverb driver require in the first place? If it is only half a watt say, then neither tube is stressed. And that is forgetting for the moment that the amp parallels the two halves of the tube for higher dissipation. I have no idea what the reverb driver power situation might be.