because steam deck is more like "PC" than a console

I know, I know everything can be a "PC" if you look close enough but hear me

people can create their own ecosystem of walled garden whenever they want

Antitrust law exists exactly to prevent companies from making their own ecosystem/walled garden that competitors cannot sell into. Product tying (forcing you to buy product B in order to buy product A) falls under that umbrella. Game console are not magical in this regard.