I’ve joined one of the Steam Dev Days conference in Seattle. It’s around time where people was still doing things like cross buy etc. (so buying the game on website unlocks both steam, dev’s own drm free version, maybe even console version.) I do not know if any of the actual developer term is updated after that time, but during the conference, one dev asked question exactly like this, can he sell his version without the 30% cut from valve if it does not going through Steam while giving away the steam key for free. The answer is no.

During the time it was explained that if you sell on different platform, that gives better sale %, steam can also impost that sale % on it’s platform. At the time EGS was still not a thing but people asked about can they have different price on different platform, I think the answer is also no or not recommended, as they can request you to match say the base price of itch.io but they don’t mind if that sale and software never use anything from steam. They specifically mention if any steam feature, like invite steam friend is used, then no, even if your game are not downloaded or use any steam distribution feature.

You are in the wrong here, Steam have a term where you can’t mark the sale cheaper on any other place, including your own website as you can generate your product keys.