The majority of the content is here, and the integration with the broader fediverse is a bonus.
I think this is a bit of an illusion that you're seeing as a result of federation being broken currently. I doubt kbin is the largest player in the entire link aggregation side of the Fediverse, but because federation isn't working at the moment, we're only seeing things that are posted here. Once kbin is working as intended, you'll start seeing content from all over the place and local content will probably be a minority.