This missed in the various greps I did before 4432e898be07 was merged. It only has one key segment passed in (likely due to using dashes instead of colons), which would not happen if WANObjectCache::makeKey() was called. The method is also needed to handle engine-specific key limitations.
Nonsense entries like "MediaWiki.wanobjectcache.centralauth_user_fff1214d22d6cde50dbe5a95f1d328ac." need to also be deleted from graphite.