I take your point, Garry, and I would agree with it. If I'm understanding Pup correctly, though, his basic point is that mixing for Atmos is different from mixing for 5.1. So the 5.1 TrueHD "core" (or, on non-Atmos-capable systems, the 5.1 "downmix") may sound perfectly fine. But it will sound fine accidentally, as it were, or maybe incidentally, but not "intentionally." That is, it won't sound the same as it would if the mixing engineer had been mixing specifically with 5.1 in mind to begin with.
Of course if you include a dedicated 5.1 mix in addition to an Atmos mix, then that's one more expense the record company has to pay for. And the reviews I've seen of Goats Head Soup suggest that "spare no expense" was not exactly the governing ethic of that release.
Anyway, sorry to parachute in to this thread so late. As someone with both a (minimal, 5.1.2) Atmos system and a non-Atmos-capable 5.1 system, I want to say that all of you guys are performing a real service here. And the more "metaphysical" aspects of the discussion are interesting, too!