I don't know, if someone knows better they can correct me, but I think there's a cultural difference between the west and Japan in how we treat "spin-offs." I'm reminded of how Koji "IGA" Igarashi was surprised by North American gamers basically disowning the Castlevania games he called "gaiden." I think some of these game designers don't realize that, to many, many players, "spin-off" means "unnecessary cruft, avoid entirely."
I mean, what marketing team on earth would willingly slap a label saying "Less Useful Than The Original" on their product? That's what "spin-off" means in the West. At least, that's what it means to half the gamers I've ever encountered!
That said, I don't understand his using it with KH in general. I know I'm playing an old tune, but does he not realize how incomprehensible KH3 will be when Sora's off rescuing characters the supposed "core story" never even introduced? And calling Chain of Memories and KH3D "side stories" even though they concern the main cast directly is just a disaster, it's doable but not this way. Your guess that it's just short-sighted marketing splurge is the only thing that makes sense!