Making something like 3dsen is a much more difficult task than being able to display a game that is using a 3d engine and so would generally be fundamentally capable of providing a stereoscopic view. That said, most of them currently don’t offer it anyway and it’s still not quite zero effort to add such a capability.

It does seem like Valve are saying they want to let you view a game in a stereoscopic 3D if the game has support for it built-in already so it would be 3d support for flat games (meaning games primarily made to be run on a screen) but only if the game has support for it. That’s good but does mean there’s still a need for either developers or 3rd parties to make the changes needed for it to function.