I'm with Buffalobiian. I don't see any evidence that the Chimera Ant's phagogenesis is any kind of Nen ability.
This is a completely semantic argument. Especially since there are settings where actual "Magic" is itself inherent to the world and the raw potential a person has for Magic is dictated by their genes.
Nen is just another generic "chi" trope. Whether you consider that stuff to be "natural" or "magical" is a pretty pointless distinction to try and make.