What is the boundary in the atmosphere seen in this video?
Yes this is neither aurora, nor reflection of any light.
This is airglow, that comes from upper layers of atmosphere.
The brightest emission is green 558nm light from excited atomic oxygen in a layer 90-100 km high. At that height most of the oxygen is in atomic state, not molecular, and is constantly being exposed to solar UV radiation.

Here you can find more details.
 airglow formation