That was a silly parody. The fact that he can play in ZUN's musical style so well tells me either he's just that talented of a musician, or he respects ZUN enough to make a silly song picking on some of the common themes and botched English in Touhou. Either way, you wouldn't know ZUN's style that well unless you've heard a lot of his music and played the games, which indicates that he probably is a pretty big fan. Honestly it seems like Undertale is kind of a heartfelt blend of Mother and Homestuck, both of which Toby is fond of (and composed music for, in context of his Earthbound hack and homestuck music), and I wouldn't at all be surprised if the bullet hell styled minigames in the battles were inspired by Touhou.
I seriously see nothing to be offended by here.