Steam =/= steam deck. And even then it's not really playing native Linux games if you argue Linux. Its just emulating (in layman's terms, I know what WINE stands for) where most devs actually make their games for.
Microsoft doesn't seem to care (or it's legally dangerous to care). Nintendo does.
What's happening under the hood (Windows translations layer, etc) doesn't really matter for this comparison of Switch to Steam Deck. It only matters what the user is allowed to do/run on the thing. On Nintendo where it's closed, the user is prevented. On Steam Deck where it's open, the user is not.
Microsoft doesn't seem to care (or it's legally dangerous to care). Nintendo does.