Rootkit implies it's trying to hide its presence. DRM software does no such thing. It simply wants to assert greater control over the hardware and restrict the user from executing some action in some way in exchange for access to something you wouldn't be able to have due to lack of trust. In the case of anticheat, many do not find its existence malicious or anti user.
Of course the well known gaming company that releases a distro is Valve. But, rootkits don’t seem like they fit their particular ethos (they are well known for their less annoying DRM scheme, right?). TBH, it seems like a rare opportunity to break the hold they have on the “game store” concept.