First of all, it's still possible, as you said. Second, we must always take humans into consideration. Let me explain.

Most people don't know what PGP is, thus you can sign your posts with any crap and most won't care.

Out of those who know what PGP is and how to use it, most won't be checking the posts as they will auto-assume that if it's signed - it's ok.

There will be a small minority of those who care, but they can be silenced easily through social engineering.

It is possible to integrate server side signature checks, but then again you're vulnerable when someone gains unrestricted access to your account.

The correct solution requires multiple features to be implemented.

1. Lemmy web UI should move to an industry standard framework, which disables XSS and other attack vectors by default. For example, Angular.
2. Official admin announcement, as well as important admin tasks, should require 2FA on each action. Like it's done on GitHub, for example.
3. Cookies should not contain JWT tokens, they should only cotain session id for read only purposes. JWT token should be moved to sessionStorage instead. It's a lot more secure by design than cookies. You can still exploit it, but it's noticeably harder.