I believe the rule you're talking about only applies to literal in-game transactions - i.e. the binary you put on steam cannot itself implement a non-steam wallet. But there's no business rule against selling in-game content elsewhere, like apple is doing.

I claimed Steam was similar. I did not claim that they share all of the same policies.

TFA is about Apple's policy on purchases that happen outside the app. It used to ban even linking to them; now it allows that but it wants a cut. Steam doesn't do anything similar - it has no rules about purchases outside the app.

Steam is not comparable because it is not a first-party platform.