Right. "Steam exclusivity" is not a thing where there's exclusivity contracts or anything like that, but rather developers "just" deciding other stores are not worth the effort right now. Both in just being additional effort to setup/promote and for smaller titles it can be important to have all initial sales happening in one place, because ranking on Steam (and other stores) is tied to getting enough sales/reviews/... happening quickly. And then some add GOG etc only later to catch the long-tail of sales there too.

(and I guess Valve first-party titles are a special case of course)