Just playing devil’s advocate here. I don’t necessarily disagree with you, but there are some interesting factors at play.

1. The Steam Machine won’t need a screen or battery, two of the most expensive components on the Deck. So that can go into better CPU/GPU/RAM instead.
2. Valve proved they can make a successful physical hardware product with the deck. That gives them a lot of negotiating power with AMD to get the best deal they can.
3. Unlike with the Deck, they’re releasing the new gadgets in almost all major countries simultaneously. That means they’ve may have already started manufacturing months ago, and are benefiting from economy of scale at an entirely new level.