I would guess there is photocell next to the LED and comparing the amount of light to the ambient light the cameras get. They might also be doing something clever and using the LED itself as a photo diode.
It has always felt under-utilized to me. Maybe it needs power-sensing hardware that's too sensitive / too fast for most scenarios / costs?
Glasses may be a use case where space is at a premium, though, so it could make sense there.