its semi-canon in the same way that the Animated Series is not considered as such - in that Gene Roddenberry said so. Gene wanted the Enterprise to be a ship with history so he pronounced that it was about 15 years old by the time of Kirk, with Pike and April being her former captains. And although never expressly stated anywhere the launch date of 2245 generally became the "established year" amongst fans and writers by the early 70s (and was hence noted in The Counter Clock Incident). Robert April was the name of Star Trek's captain in Roddenberry's original premise before he changed it to Pike, then then, of course, Kirk.
Another instance that made April AMOST canon was for Archer's bio which was prepared for "In a Mirror, Darkly" which was written by Mike Sussman, I believe. Among other tidbits was that Archer died the day after the NCC-1701 was launched under April in 2245. It just so happened that the way the scene aboard the Defiant was shot excluded a screenshot of this particular bit of info, although did kinda exist.