If you can log into the site, it’s done.
If you try to access the site and you get a “startup log” page, it’s still ongoing (and it will show you there what it’s doing).
https://jellyfin.org/posts/jellyfin-release-10.11.0/#startup-ui-and-log-viewer

I think the feel from the Devs is that there isn’t enough new functionality to justify the major version bump, this primarily being a reimplementation of existing features.

BUT, I agree with you, it should definitely be V11 under the semantic versioning scheme.
Whilst there is a migration path here, the database changes under the hood alone are likely to break backwards compatibility with all plugins (with in-house plugins being upgraded in sync).
Such breakage is kind of the defining characteristic of a MAJOR version.