As I understand it, the term "NPC" was in reference to the the MSM/Antifa/BLM, or whoever, only having a small snippet of dialogue on a subject; so weren't able to deviate and enter into a debate on it. I guess the term was deemed applicable due to this limited amount of dialogue, and a tendency to repeat it over and over themselves, or as a group en masse.
I think there are a few clips on YT, showing 'anti Trump' news outlets, all chanting the same chunk of dialogue, almost perfectly in sync - and though it could simply be clever editing; the message is exactly the same with no deviation - like an NPC in Oblivion or something.