"Then they can boost older content from the hosting site." No that's the problem. Like you yourself said back catalogues arent fetched. They can't see the older content to be able to boost it, they'll only see new content.
but relevant users cant see it, its never fetched for them to see it. Sure users on the home instance can see it, but they're on the home instance, it's already fetched for them. Ive run into this problem on here, where there is a lot of content on other instances that isnt visible from kbin. I have the option of visiting the home instance to see it, but it takes me completely off of kbin, I cant boost it from that page.