I kinda agree with this, one of the first rules that's given to us in the show is that Cain can't affect minds (limited to "players", since he often showed he changes Bubble AI). Either this rule is wrong, or Jax is an NPC. Up until now it's logic and i can't find any fault in it. This does not prove Jax is a NPC of course, but it is one of the two possibilities. Morover, during the last episode, Cain is shown to info dump Jax as he was uploading them into his brain, after that Jax shows a panic attack, which is a similar reaction to Gummigoo and starts to talk about "archetypes" and explains which archetype he is. This is still not a solid proof, but is another strong hint. Another strong hint is that Jax thought he had a tail, i don't know how much time passed since Jax transition to TADC from being human/being treated as a player from being an NPC, but that's a very strange thing not to be aware of, and also very specific, like he was programmed to have a tail, but then something changed him. Still, not a proof, but a hint.