I wouldn't call Castlevania II at all in the metroidvania style. It's an open world, but outerworld, and you go into different dungeons. More similar to Zelda II in my opinion (minus Zelda II's overworld map screen you traverse, of course).
As for the OP, they just wanted to try something different, I guess. But even if you consider them stealing that idea from Metroid, how many platformers stole their ideas from Super Mario Bros, you know?