Mastodon the software isn't. Mastodon the community is because they want to control who is and isn't allowed to federate.
iiuc your argument, it would be more accurate to phrase it as: mastodon, lemmy, and other federated social protocols are like archipelagos of small islands.
Or, you were referring to bluesky.
Mastodon contains several sub federations that near automatically ban each other’s users… So a very tribal and extremely violent archipelago of small islands…