Breaking out of a container might not only be achieved by root processes or (ab)use cases of SETUID/SETGID, but through risky bind mounts of the host file system, too.
UID 0 might help with additional permissions in such a scenario, but i'd argue it would still be considered a separate point.
The docker docs even acknowledge it here (search page for "security implications").
Side note: Most english sources call it container breakout instead of container outbreak.
Using your favourite search engine with both terms will demonstrate the difference in search result quality.