Lies.
Steam not only provides 5000 free keys of your game as the Default Release Steam Keys, no questions asked, they also provide free keys in case you're selling your game outside of steam. To quote the Steam Key Rules and Guidelines, "You should use Steam Keys to sell your game on other stores in a similar way to how you sell your game on Steam. It is important however that you don't give Steam customers a worse deal than Steam Key purchasers".
The only reason you might not want to give people on itchio steam keys is that it would force you to lower the Steam price of the game to the same level it was on itch, since having it cheaper outside of Steam would violate their guidelines - and you clearly want to squeeze out as much money as possible, while lying to your customers and asking them to buy the same game twice.
