I discovered this as well. Not only, they're faking it with the new YouTube design as well. I had to set my user agent to Chrome to get the new dark theme (it may have changed later).
I did try different user-agents using AgentX[1] and with Edge UA they said the new look wouldn't work (or in effect something along those lines), so there is not much actual feature check going on. Only setting Chrome would give me all features.
Google is doing what Microsoft did 10 years ago - it didn't go so well for Microsoft when people eventually caught on.