I don't think content creators can choose not to have ads in their videos. Creators can turn off monetization, but YouTube can still play ads, they just won't pay the creator.
I do agree with your general point. Creators can choose to show less ads, and we should prefer channels that choose to monetize less.
EDIT: I'm no longer sure whether YouTube is displaying ads when you do this. The wording I found in their help pages is ambiguous. I think they reserve the right to display video ads if monetization is off, but they are not displaying ads (currently).
Well now I'm intensely curious about this. I've turned off monetization on this video https://youtu.be/liJac6RysE4 . Please let me know if you see any ads on the video, I'm curious if YT will still display them regardless of my input. (So far I'm not seeing any ads).
Edit: Also, when I click on the help icon next to the monetization bubble, the entire Google help article[0] talks about turning on ads. Which seems to imply they're off by default, but they don't mention turning off ads, so who knows if that's just "clever" writing to indicate there's no way to turn ads off.
That said, I checked the parent's video, watching it all the way though... and I also didn't see any ad (and I even watched it on my Chromecast, which usually triggers a lot more ads on Youtube, which is a reason why I never use it for Youtube, but only for Netflix, Disney+, etc.)
I assume that ads might be shown, but only rarely. Probably Youtube wants to run them, to have the flexibility to recoup on some of the infra costs for videos which wouldn't otherwise earn revenue (i.e. they aim net zero for non monetized videos... but this is my speculation)
Partners can choose to disable all types/placements of ads ("skippable video ads", "non-skippable video ads", "pre-roll ads", "mid-roll ads", and "post-roll ads") except for "display ads" (that is, banner ads). As for whether those options actually work, I can only assume so, but the article you linked is specifically about non-partners.
Another point is that the algorithm (which decides what videos are shown on the homepage, sidebar recommendations, notifications, etc. and provides the majority of views to videos) essentially ceases to promote at all any video that's not monetized.
Which I think is fair -- after all, I think they're already doing non-monetized creators a favor by footing the entire bandwidth bill, they don't owe them free promotion!
But it obviously puts a limit on what a creator can do about ads if they are actually trying to make money (even if they don't want any Adsense money and only want to do their own in-video sponsors, or heck, even if they're just looking for "exposure"). If they turn off monetization, they'll drop off the face of the Earth, essentially.
Hence why creators care so much about a single demonetized video, basically it means the video goes into a black hole and earns them basically no views. It exists, but it may as well not.
Many videos feature music that is not free-to-use. Music rightsholders demand their pound of flesh on those regardless of what the creator says. "let's play" videos (where people upload and comment themselves playing) is a gray zone, if only because of the music rights, which can cause ads to show as well.
https://www.makeuseof.com/youtube-ads-without-paying-creator...
I do agree with your general point. Creators can choose to show less ads, and we should prefer channels that choose to monetize less.
EDIT: I'm no longer sure whether YouTube is displaying ads when you do this. The wording I found in their help pages is ambiguous. I think they reserve the right to display video ads if monetization is off, but they are not displaying ads (currently).