You may be overthinking the idea. Referring to someone as an NPC is just a shorthand to describe people who fit into the crowds, who don’t make real impressions on you, who really don’t have anything to contribute to your life, but who likely contribute a whole lot to the people in their lives.
You’re an NPC to almost everyone, just as everyone is an NPC to almost everyone else.
You can’t always drill down into people’s personalities to give them real unique identities in your mind.
It’s like walking into a crowd during a concert. Everyone else is an NPC to you except the couple people you went with.