There are channels that have direct sponsors, I think sometimes the deal with the sponsor is to not monetize the video, so if I get sponsored by Intel I would have to turn off the ads in case an AMD ad would show up.

This is a speculation of mine from what I observed on some tech channels I am not familiar with YT Terms and Conditions.

If you get third party sponsors and not allow monetization is YT getting nothing? If yes then we can see what they are trying to make here.