There is a clause in the Steam EULA or TOS or something for publishers/developers that states you can not OFFER STEAM KEYS for less than the price you sell the game on Steam.
It absolutely does NOT prevent launching your game on 2 different platforms at diffrtent prices, so long as any secondary stores are not offering a Steam key to own it on Steam.
Dipshit Sweeny is basically illiterate.
The only "proof" that it really is exactly as Sweeny describes, is the word of the dev of Overgrowth, a known liar and scammer, who has been in a legal battle over his claims that Valve delisted one of his games over a price difference on anothet store for years now with nothing being proven.