I think that’s somewhat of an oversimplification. The point of the instances isn’t to be interchangeable, it’s to provide decentralization and lots of choice to Lemmy users. Lemmy itself is more or less just a protocol a bunch of different link aggregator sites are running on, each of which has its own administration style and community policy. There are lots of general-purpose Lemmy instances, lots of more niche ones, and a bunch of small/personal ones that don’t have a strong identity yet, and all of them together form the Lemmy ecosystem.