There is also the other point of view, that every other human that is not you IS an NPC, we can only prove that we exist, and even then that is muddy, for all we know everyone else is just a figment of our imagination, we can neither affirm nor deny it, so it is not wrong that since the only one that you can affirm to some extent is a player character is yourself, to call everyone else an NPC, as you have no way to prove they are not an NPC, you can't see what they think, you can only assume that like you, they think and they are.
And yes, I am being pedantic with a philosophy I do not even endorse nor fully understand, just wanted to be a contrarian.