I know at one point they were saying it was not canon but apparently that has since changed. It might not have many ties to the MGS story but that doesn't mean it's not canon.
There are two universes, the canon one and the one where GB and the Acid games are. Not everything in the canon universe has to be called MGS or tie into that main story in some way. At least that's how I see it.
I agree with your sentiment that Kojima would probably want to keep any big advances in the story for himself but, this game doesn't look like it's going to be anything more than just a fun little sidestory set in the same world so I don't think that's an issue.
Of course, there's always the chance that he retcons Rising away in a future game anyway, making this entire discussion pointless. Videogames, yo.
Here's an interview with the Creative Producer from KP saying it is canon with MGS:
https://www.youtube.com/watch?v=d7BTtiAQuZ0