I could find no link in the DC database to support your assertion that Imaginary Stories are synonymous with Elseworld. I also found no information to categorize
The Dark Knight Returns as an Imaginary Story. However, I found the following information:
"'The Dark Knight Returns' is not considered a part of mainstream continuity, and retroactively labeled an Elseworld story."
http://dc.wikia.com/wiki/Batman:_The_Dark_Knight_Returns_Vol_1
"
DKR takes place in a timeline outside the continuity of the DC Universe, but is still considered at least partially faithful to the source material and Batman mythos at the time it debuted."
http://dc.wikia.com/wiki/Batman:_The_Dark_Knight_Returns
The quotes beg the following questions:
Who decides to retroactively label
DKR as an Elseworld story?
Who establishes
DKR to be outside the continuity of the DC Universe?
Who gauges the level of faithfulness to the source material?
As far as I know, wikipedia type databases comprise voluntary information from secondary sources and that makes them highly subjective, arbitrary, and unreliable. Now I understand why college professors strongly discourage using wikipedia as a reference. However, we can use the database as a premise for the discourse. Imaginary Stories becomes a non sequitir based on the information provided by the database. Furthermore, the internet did not exist in 1986 as we know it today and for this reason the contributor or source abitrarily and retroactively categorizes
DKR as an Elseworld story. I am unaware of any baccalaureate or doctoral programs for categorizing comic books and canonizing them.