Twitter auth is already working but not activated in this instance of Phabricator. Please activate it as an external auth source.
Since Twitter Plugin is "ready" and working, I see no particular reason to not enable it for this install :-)
I use twitter as my default login provider, usually. So it would be supportive to be capable to make use of it here for me, too.
Also, having it working in this install would raise the notification level when it is broken due to changes at twitter or such.