What would happen if kbin's admin (ernest), or any other admin of a server( by that I mean Mastodon, Lemmy, not an instance of kbin) one day decided he's going to pull a spez?
Yes, the code for Kbin, Mastodon, Lemmy, etc. is open source, so anyone can spin up a new instance if they want. However, spinning up a new instance of Kbin would not save the content of any other instance if it were to go down.
If kbin.social shuts down tomorrow, then all the communities and user accounts that are hosted there are gone too. Kbin would still exist because there are other instances, but nothing @kbin.social would exist, if that makes sense.