It's a side story more than anything else if you ask me. I'd probably place it into the same category as the movies. It's a what-if. As it's not a part of the series proper and doesn't quite fit in with the events we're given, I'd personally leave it there.

A large, projectile-throwing weapon? Nah...
But as canon to Dragon Ball manga? The manga version, yes I consider it, like the JSAT manga adaptation. But since Dragon Ball never had an official canon, it's up to you to consider it canon or not. Sueisha however introduced it as a direct sequel to the original special rather than a what-if.