Currently, I believe we require "Differential Revision" to strictly have recognized, full URIs. Instead, it should accept full URIs or no URI (it's possible it does actually do this, but we should verify that's really the case if so), and only reject full URIs for unrecognized domains.
This is a blocker for eventually making the full URIs optional.
In modern Phabricator, we have tons of unqualified references (like "Fixes Txxx") so qualifying a tiny fraction of the references isn't really useful any more.