It should be noted by the way that any developer can start making VR games for the Switch now. They don't have to wait. The second a third party HMD is made, anyone can make VR games for it without Nintendo's consent.
Also, just for clarification, the Switch screen itself does have both an accelerometer and a gyroscope that is independent from the ones in the Joy-Cons, according to the official spec sheet. So the HMD wouldn't need to add that tech.