You can add me to the list of people who were surprised at the ending--one all-but-guaranteed to make this an "imaginary" story for every universe in question.
I consider that a given right off the bat for any crossover like this. The "alternate universes" excuse doesn't really work for me, since alternate timelines should still have the same laws of physics, yet different SF universes tend to have rather different physics, not to mention cosmologies. Not to mention that they sometimes have a tendency to be mutually fictional (for instance, Star Trek exists as a TV franchise within the Doctor Who universe, not to mention the Marvel Universe and probably the DC Universe).