I don't agree with this at all. If you break up youtube from google, I think people will treat those very separately - they serve very different purposes. Same with facebook and instagram.
People already treat them very separately, I'd be surprised if most of the public (not HN) even knows that Google owns YT.
I assume the GP meant "the people in charge of..."
YouTube is probably like 8% of Google's revenue, not to mention that even if it became a separate company it'd probably just serve Google's ads. And it's the only domain other than the main one that matters enough to separate.