The community belongs to a website, yes. You're just subscribing to it remotely.

Lemmy is decentealized in the same way the web is decentealized. You can't get articles from Blog 13705 if blog13795.net goes offline. That doesn't make the web not decentealized.

At the end of the day, the whole fediverse is a bunch of independent websites hosting copies of other websites' content. They're not cloud communities, they're mirrors.