The documentation says the pg_cancel_backend role cannot signal "a backend owned by a superuser". On the contrary, it can signal background workers, including the logical replication launcher. It can signal autovacuum workers and the autovacuum launcher. Signaling autovacuum workers and those two launchers provides no meaningful exploit, so exploiting this vulnerability requires a non-core extension with a less-resilient background worker. For example, a non-core background worker that does not auto-restart would experience a denial of service with respect to that particular background worker.