Developer Experience I suppose
But in the spirit of "there are only two business models - bundling and unbundling", I guess there are only two marketing tricks: use an old term for a new thing, and introduce a new term for an old thing...
Take that payment system. People who buy things are obviously users, people who write CMS plugins for that system are obviously developers. But what about, say, analysts studying reports from that system? Accountants making sure the money flows where it should? Sysadmins keeping the backend components running? These are all users too.
Where UX makes sense as a broad concept, giving a separate acronym to one small subset of potential users... doesn't make sense.