In my opinion, a "diorama" is simply a snapshot executed in three dimensions, rather than just two. And as with all snapshots, the "story" is in the mind of the beholder.
Ted's Sherman dio is most definitely a diorama, as it is indeed a three-dimensional snapshot of a scene. And it indeed tells a story. Maybe not a blatant, in-your-face, here's what's happening A-B-C kind of story, but a story nonetheless. At the
very least, it says "Sherman at war, somewhere in Europe." My mind can fill n the rest. I don't need to be spoon-fed everything!
In my opinion, once you put a model on anything more than a blank piece of wood, you have a diorama. Actually, a model by itself is certainly capable of telling a story, so if you simply define "diorama" as a "display telling a story," then I believe you would have to include stand-alone models as well. It's all about imagination and interpretation, folks.
Just my opinion, and not a slag against anybody, but looking at any diorama and claiming that it has no "story" at all would seem to indicate a lack of imagination on the viewer's part.
Perhaps this is all just a matter of semantics, an argument supported by those talented individuals who, moreso than "model" dioramas, feel that they "author" them. Granted, some dioramas do tell more detailed stories than others, as do photographs of actual scenes.
Which is fine with me, because I enjoy them all, whether the story is only vaguely suggested, or clubs me over the head!
It's all good!