Haha it's a weird one! The person who suggested metroidn't said "Either you metroid or you metroidn't". There's been a couple of interpretations in the Discord server, one being to focus exclusively on the Castlevania side of things (since metroidvania is a combination of both terms). Some thought about a reverse metroidvania, some others as having a setting on a seemingly opposite side of Metroid... There's many ways to interpret it! I should point out that choosing metroidn't still requires you to make a metroidvania, and you can pick another or more themes to mitigate this one.