User blog comment:Martialmaniac/Inferred Nen Type/@comment-4932025-20160620002148/@comment-4932025-20160620191510

"So, in Meruem's case, we are considering the card game to be the source? "

Yeah, it's official material and gives us something to fill those unknown Nen types. I don't see any harm in it as long as we reference that it's from the games. It makes sense for Meruem to be a specialist tbh.