He's not the supervisor.
Other supervisors include people from Capcom, Sega, Konami, Platinum Games, Nintendo themselves...the "Original Game Supervisor" credit most-likely means they're supervisors of character/series portrayals, so Kumizaki would be supervising the Kirby stuff.
I don't see how this could be a hint, when he always seems to translate his first sentence somehow - meaning what he typed for "Hero" would be "brave" in English due to how it was translated - and all it proves is stuff we've already known. And if it was intended as a hint, it'd be to the point where he risked breaking an NDA due to how obvious it is. Dragon Quest is incredibly popular in Japan, he's just doing what people normally do online and posting stuff he likes.
I still believe "brave" is most-likely Erdrick, though, but this ain't it. Really, "the Brave's shield" brave isn't even a noun, this is Japanese getting translated.