“Android” one can also include “Daydream” or “Gear VR” and in the future more, because its android (Aka, Fragmented) and thats probably the only reason currently High Fidelity wants to go for Mobile VR. iOS doesnt.
If you want an iOS app, using OpenGL-ES will not work as great as a metal one, since metal is basically a quentessencially optimized for the iOS devices. Considering iOS Devices rely on this more than hardware power (unlike android) its quite reliant on software to be written ontop of it to get the best experiences out of it.
Sure you can write for iOS (or even Android), but it doesn’t mean it will be usable for a long time, especially considering that the interface still requires one to have quite beefy machine to even run at 60fps HD (and not per say at the stupefying resolutions the modern phones go at).
IMO: Better get the engine running better, and optimized better and perhaps looking better, before trying to cram it into the phones. Or just have a text / voice client with minimal features, but generally speaking I dont know if anyone has the time or money to do so at this moment, without and proper revenue yet.