It's alright to disagree. :-)

You can’t advertise a set of criteria and use another one.

I don't think Valve does that.

Why would they advertise it must perform well

It does not advertise that. Valve tests the games and according their ruleset the game is Playable or Verified. It does not mean the game is playable in a good state for everyone, because the quality of the game is not what the verification is about; it is about being compatible.

and then instead use a comparison with a windows system

Because that is what the compatibility is about. And the system with Windows is just to understand the point of the verification.

We disagree on what the bare minimum acceptable performance is

We do, but we also disagree on what the verification is about; the performance is not that important.

Frequent deeps to 15-20 FPS is playable but not enjoyable

Depends on the game. You still ignore that. Lot of games are still enjoyable at 15-20 FPS constantly.

Certainly not something I would say that performs well.

Verification is not about the performance of a game. See it like a PS5 console, if a PS4 game runs on the PS5 system the same or better through backwards compatibility, then it is verified. The game might run at 25 to 27 FPS most of the time, sometimes dip down to 23 and 19 FPS (in old PS3 days this was happening with games) on the Playstation 4, so if it runs the same on Playstation 5, it would be verified. That is the sort of thinking on the Steam Deck verification.