To me the problem isn't that flat earth videos exist on Youtube, but the implied "legitimacy" of the recommendations, along with the typically addictive UI elements like autoplay. Flat earth / Nibiru / etc insanity has always been on the internet, but when you saw it, you knew that you were in some obscure corner of the internet that was not normal. If anything, YouTube's insistence on mixing brand-safe, front-page recommendations along with content-adjacent recs lends credibility to the (many fewer now) "rabbit hole" recs. If you're on "Tony Blair is a reptile.mp4" and all you see are things like "Tony Blair is ACTUALLY a hologram.mp4" with 2k views, it's like "whoa, this is weird". But if you get 3 reptilian recs and 5 TED talk recs that have millions of views, maybe it seems more normal? I'm not sure.
Source: an analysis of 40,000 trending videos [7 months of trending tab videos]: https://www.youtube.com/watch?v=fDqBeXJ8Zx8
An awful lot of discussion around YouTube and human curation/moderation elides just how much content they're dealing with; the most recent figures I've seen -- which are over a year old at this point -- show that about 500 hours worth of video is uploaded to YouTube every minute. If we figure an average video is about 15 minutes (a 2017 statistic I found suggested that "the average length of a first page YouTube video is 14 minutes, 50 seconds"), that's 2000 videos a minute -- over 30 a second.
Is it impossible that YouTube employees have secret levers to raise and lower the profile of individual videos and channels? Of course not. But it's absurd to think that such manipulation is common, let alone corporate policy.