True right now they are compatible, however it seems like that Facebook and Valve are on a collision course over what they think the ecosystem should be (and who is going to be the "winner").
Sure, Google has a Play store and a competing OS, but it's not like you can't find Chrome in the iOS store.
Different HMDs are arguably more similar than different GPUs, and they manage to support the same software, but in the early days there were plenty of Glide-only games. That didn't work out so well. Most people will probably just use tools that optimize for any popular HMD with little enough effort that only contracts will make exclusives make sense.