At higher light, the tube brightness is adjusted. Usually by autogating, but ABC is also active.
At lower light levels, the tubes (all manufacturers) start to operate in analog mode - this is useful with a luxmeter for estimating the local albedo levels.
The gain is entirely relevant once you get into this range, and you are limited by the 2700~2900K light source, which doesn't really match night vision in the IR region, though is a reasonable fit for Gen3.
Your eyes can compensate for a fair bit of gain difference, so brightness is a curious measure. once you get about 30,000 times, a tube is suitable for military applications.
What this experiment doesn't do, and what a Hoffman tester can't do, is replicate the night sky illumination, which is what really drives "in the field" testing.
But it's a really cool test nonetheless and I liked the video.