It's a spinoff that doesn't contradict the storyline in the main entries of the series. Fans can therefore consider it "canon". But I wouldn't get too hung up on the idea of "canonicity", as it's never really fit MGS.
Series creator Hideo Kojima called it a "parallel continuation". That doesn't mean it's non-canonical, only that it may differ from what he envisions of a post-MGS4 future. In that sense, it doesn't follow a "Kojima-canon" (for lack of a better term). But at the end of the day, it's immaterial.
As I posted above, the poll could have phrased the question better, by referring to these games as "spin-offs" instead.