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.

I wish I could find more things talking about this, but there's this classic demo of the technique: https://cs.nyu.edu/~jhan/ledtouch/index.html

It has always felt under-utilized to me. Maybe it needs power-sensing hardware that's too sensitive / too fast for most scenarios / costs?

It's actually not that hard to use LEDs in this way. I think it's not done a lot because in applications that need a light sensor, a purpose-built light sensor works a whole lot better, and it's rare that space is such a premium that having one is an issue.

Glasses may be a use case where space is at a premium, though, so it could make sense there.