These are pretty calm messages to an Australian and Garry is British, so culture checks out.

// What the fuck
// Fuck dynamic compiling.
// what the fuck is this shit
// What the fuck, why isnt this a method

Should this by the by commentary be there?
Not really.
But as a programmer, I understand each and every time I see something like:

// Urgh this is so dirty, Invalidate() and Refresh() do nothing.
tButt.AutoSize = false;
tButt.Width = maxWidth;
tButt.Height = maxHeight;
tButt.AutoSize = true;

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.