It is a monopolist in the format it specialises in - medium length 'creator content' that the creators typically post every 2-10 days. Some do post to Nebula and Patreon, but really, there's nowhere else to go for that kind of content, and that's the content that most of their ad revenue is attached to.
How are Netflix, Hulu, Instagram, Tiktok, and Twitch compared to YouTube? It doesn't make sense, they aren't the same niche, you won't find Numberphile, 3Blue1Brown, on those platforms, you won't find reviews of appliances, tech, nor tutorials for how to fix your dishwasher, etc. on those platforms.
YouTube has a whole vast amount of independent production (and some now independent-looking but owned by private equity) which it has cornered into the platform, nowhere else you can find the sort of content that exists in there.
You are just conflating "streaming video" into a single homogeneous market, it's not the case.
I've definitely watched repair videos on tiktok. And one of my favorite (indie) tv shows was only on YT for some reason instead of Hulu or Netflix. My kid watches videogame playthroughs on YT, not twitch. And that's completely disregarding you can listen to music on YT.
When defining a monopoly you can't just say "only this subset of the market is the market we're considering" you have to look at everything it does. As the FTC just learned
Your theory of
> just a way for some product manager to fluff up their metrics for a promotion.
is the most likely culprit