I think you could see it fitting, but I would otherwise think of it as a separate continuity. After all, Batman Year One is not considered connected to Dark Knight Returns other than being Batman and written by Frank.
I think I remember reading that Miller considers everything he did with Batman part of the same canon. Like that in his eyes Year One is the origin of Batman for not just one continuity track but for many others, which includes the DRK Universe (and Batman/Spawn).
I think I also remember reading that he said that he considers Year One, All Star, and DKR a sort of trilogy.
Did they ever release the final six issues, the ones that were no longer All Star comics but changed the title to boy wonder or something like that?
"Comics are like a Rorschach test; everyone has a different opinion on what they are and can be"