SHPD17 why would you enable an extension that adds a signup button if you disabled sign up though ? 🤔
Does it just show the signup modal or does it actually lets you complete the registration ? As this extension only adds a button and doesn't change anything to the backend, signup should still be blocked at the API level ("You don't have permission to do that") if you try to complete it while signup is disabled in the admin.
If you indeed can still register, it looks more like an issue with core that wouldn't properly enforce the closed signup.