People are just obsessed with turning everything into A vs. B, and they'll argue this shit til no end even if it makes no sense at all.
I think it's more that some people are obsessed with inventing new completely pointless genres when those bands already neatly fit into existing and more well-defined ones.
Viking metal is just as pointless as calling Agalloch "forest metal". Sure you can call that a genre, but why does it need to exist when it doesn't really give you any indication of the sound of a band that belongs to it (which is the case with viking metal, since bands from folk metal, black metal and even melodic death metal are lumped into that 'genre') and when there are existing genres that the band already fits into that do?
There's just no need for it to exist. Same with forest metal, battle metal and probably a few dozen other genres that people seem to have conjured up for no reason.