Just because it's an out-of-continuity crossover doesn't mean it isn't a crossover. There have been other cases of crossovers that were not acknowledged in the continuity of one or the other of the participating shows.
Indeed, there's even precedent for a spinoff series altering the continuity assumptions of the backdoor pilot it's based on -- for instance, when Robin Williams first appeared as Mork on Happy Days, it turned out to be a dream at the end, but the character was popular enough that they retconned him into reality and gave him his own show.
Bad example. Mork was always real. He only made Richie think
it was a dream. Remember, after Richie wakes up, Mork shows up at the Cunningham's door pretending to be somebody (a guy asking for directions or something). After that we see him making his call to Orsen.