fascistcrusader
Restricted
Even if that were the only reference it would prove it canon. Seeing as how Gaiden are spin off stories that may or may not be canon, this statement means the game is definitely canon because making it a spin off with ambiguous canon status would make the game too distant. They wanted to make a game that was part of the main series.
However, Takeshi Arakawa, the game's director, also said this on the blog:
"As a rule we were extremely careful to treat DISSIDIA FINAL FANTASY as a genuine entry in the FINAL FANTASY series."
So the producer says it isn't a non canon spin off and the director says it's a genuine entry in the main series. Even without these there's still the fact that Dissidia is to FF I as Crisis Core is to FF VII, and the Ultimania speaks of it as canon, going so far as to say how it ties in to the other plots.