Plenty of Youtubers with millions of subscribers regularly advertise their content on other video platforms such as Nebula. Doesn't seem to be an issue.
At least for the creators I follow, they use alternative platforms for additional content (not replacement for their YT videos) for things that don't perform so well trough the YT algorithms like long form, in-depth videos that are less advertiser friendly than pure infotainment, and e.g. music educators doing song analysis which is too risky to do on YT although to my understanding it falls perfectly under the fair use doctrine.
I don't even see them as competing platforms, they offer very different products via the same medium. Legally, I don't know if having such ToS would be a problem, but as a user I would find it extremely frustrating to not have to ability to discover content that YT seemingly doesn't want to focus on itself.