I've not seen this before but it was strange.
An ad loaded at the end of a video, so I paused it. What caught my eye was the background was moving when I moved my phone, which turned out to be the room I was in. The ad was overlayed on whatever my camera was looking at, but the ad appeared stretched from a single point in the middle of the screen, which was even weirder.
Edit: The ad was using the rear camera, not the front facing one.
I've looked through my phones settings and there are no options to toggle YouTube's camera access either, so I feel like it's safe to say this is being forced on users (surprise /s).
Needless to say, that app is no longer on any of my devices :)
I agree with your logic but when I reinstalled the app to see if I did allow camera access, the only prompt that came up is to request permission to send notifications.
Then again, it was logged in with a Google account so maybe it has something to do with the account permissions? I'll be able to investigate that later tonight to be sure but even after logging out and a fresh download, it still prompted for only notifications.
Regarding the glossy screen, it was displaying from the rear facing camera. I apologize for not mentioning that in my post but I'll update it to reflect that.
Edit: typed front facing instead of rear facing