> Isn’t sponsor revenue ad revenue?

It is, but it's functionally different because the content creator you are watching is both directly getting that revenue and often doing the testimonial for you. They have an incentive to avoid being annoying about the ad as it reflects bad on them if they go nuts. It's also usually a lot easier to skip. It doesn't capture your video playback and force watching.

The money you get from youtube make things ambiguous. Especially if someone is watching your stream with youtube premium.