It wouldn't surprise me if it was only compatible with more recent VR sets. They are giving a shot at an engine that could set the pace for the next decade, the same way CryEngine 3 and Unreal Engine 4 couldn't run on current generation average rigs when they came out.
VR sets are really just display and IO devices, there's no good reason for them to not work with older equipment. That's not to say that Valve isn't likely going to make best use of their Index Controllers to provide a unique and purpose designed experience which would just be represented by a button press on other sets. Just that they wouldn't be any good technical reason not to be compatible.
Gabe Newell said that Valve was jealous of Nintendo being able to design their hardware and software together. Expect Knuckles/Index to be a major factor in the gameplay.
The new controllers are compatible with the Vive, and sold separately, so that leaves an option open. I would still be very surprised if Valve locked the game to the Index.
