I've asked Poyoyon Rock on twitter if he was Kapipopo, and he said he isn't. I then asked Kapipopo in Pixiv if he was Poyoyon, and he says he's just a fan. That would explain why Kapipopo's art looks somehow not as great as Akio's work.

Also, the lines are different between them, Poyoyon lines are different width (looks like he draws them by hand), while these are digital lines all the way...

I know Akio Watanabe _could be_ Kapipopo and still deny it, but I really don't think he is.