Several cast members from other Star Trek shows, particularly Star Trek: The Next Generation appeared in Deep Space Nine.
Ira Steven Behr said Benny Russell would’ve only been dreaming up Star Trek: Deep Space Nine in his explanation to attendees of Star Trek Las Vegas (via Trek Movie), but there are several callbacks and references in the series that would only be able to be explained as part of his imagination as well.
Of course, at the end of the day, the entire franchise is a work of fiction, but it sounds like Star Trek’s head honchos weren’t interested in the showrunner’s idea becoming canon.
On top of that, the show featured a score of alien races and plotlines that tied into prior shows to the point that calling Russell’s imaginative storyline a coincidence would’ve been a little far-fetched.