I don't see that as exclusivity, more like a form of DRM enforced upon the game's copy to ensure.... whatever it is DRM meant to ensure when it's placed upon the game. It sucks, but it's nowhere near the context of exclusivity we're discussing right now.
Exclusivity, by definition and in context of video games, is basically when you
can only get them from one place. Prime examples are console exclusives, like Bloodborne where you
can only get it from PS4 Store, and only from there alone. Meanwhile, from what I know, New Vegas was available on other sites such as
Green Man Gaming. And since I didn't know better, I'd assume the
access to New Vegas by way of GMG and other sites is available since Day 1. This is thanks to Steam's ability to generate keys, from which devs or publishers can distribute it somewhere else and from what I know Steam
doesn't take even a single % cut from keys being sold on other stores/sites like GMG.
Requiring Steamworks isn't exclusivity, since the game can still be bought anywhere else. It's basically a form of DRM, and as far as I know imposing Steamworks upon a game is
100% devs'/publishers' choice.