I see you've written a book on PLATO -- I'd love to know some of the examples of these early MUDs.
I believe the deal with the noted jurist who claims to have invented email is that he may have had an earlier version of a text delivering system called email, except that there is very strong disagreement that his version of email has any living descendants.
If we assume that he's wrong, then the right inventor of MUDs is the one whose creation has many (more popular) descendants.
I played on the original Essex MUD in the early 80s, and it had a lot of distinctive qualities that directly inspired many of the other MUDs, including I believe LambdaMOO.
And, unless you have any competing evidence, I think it might be fair to say that Bartle and colleagues predecessors at least coined the term "MUD".
I see DVW discusses PLATO's multi-user games in the section "The Fifth Age", at least Orthanc and Oubliette pre-MUD1 (Essex MUD; 1978), saying "the interaction it allowed between characters was very limited; it was almost there, but not quite".
Were there other world games on PLATO in the 1970s?
(Same section adds "In late 1979, the first ever fully-functional graphical virtual world was released [on PLATO]: Avatar.")
Bartle was unaware of the PLATO system, which had numerous MUDs up and running and mature by the time his was released online.