The few sales there's been on Itch prior to launching Steam isn't going to sway the needle in any way, I assure you (I would wager it's already sold more on Steam than on Itch). If anything it would have a positive influence on sales since people that get keys would most certainly post positive reviews for the game. And that's everything on Steam, wishlists and reviews is what drives their algorithm.
If the choice is between giving back in a very minor way to the people that believed in the game before it came out on Steam, which lets face it, is *the* platform people want their games on. Or trying to squeeze every little cent out of people making them have to buy it twice - then yeah, that's scummy. No other way to say it. Dev should get their priorities in check, this ain't it.