As far as I know, this only applies to Steam keys: developers are allowed to generate Steam keys for free to sell on their website (Valve does not get 30% of these sales either) with the restriction being they cannot be cheaper than the price on Steam
I don’t think there’s ever actually been any proof that Valve disallows selling games for cheaper elsewhere as long as you’re not selling those freely generated Steam keys
I don’t get the whole “this game isn’t even good, there’s no way it was the best selling game of the year without rightoids overwhelmingly buying it simply to own the libs” idea when year after year after year the best selling games are Call of Duty X+1, Madden X+1, and FIFA X+1
A game doesn’t need to be good to be popular, and I’d bet probably 99% of the owners are just normal people who aren’t even aware of the controversy
This is true and public knowledge though as I said (details seen here in the “Steam Key Rules and Guidelines” section), if anything Valve is giving devs a lot of leeway by allowing them to do that at all, not only are they giving up their 30% cut but are also then distributing and committing to updating those copies of the game for free