It's because you otherwise get loads of self promotion spam by people only interested in promoting what they have to sell, who don't otherwise participate in, or care about, the community.
Someone who makes youtube videos about Foo belongs to the Foo community, but they can't post on /r/Foo. Why? Because /r/Foo isn't the community of "Foo."
The "community" in this case refers specifically to a group of people on Reddit.
That is, the guy will never belong to the "community" because he is busy making videos on Youtube instead of posting comments on Reddit. And you can replace that with anything else. Someone who writes their own blog, or own books, or has an Instagram, TikTok, or even a Facebook page.
He would be considered a "high effort" OC creator in /r/foo if his videos were hosted on Reddit instead of on Youtube. That's how silly this whole thing is. This is not about "the community." This is about someone's personal Reddit group that they're moderator of.
Spam is a problem and has been since the dawn of the internet. Efforts to limit it will always affect innocent people to some degree but it's not really feasible to not do anything. And reddit especially has an astroturfing problem since it became such a popular source for recommendations. Like, just looking at GP's profile here kinda adds some context as to why they might say this change makes reddit useless to them.