When he logs out the NPCs around him only then realise they might be NPCs and start freaking out, hoping that they too can log out. Because if they can't then they're not real...
That's why in Star Trek the holographic NPCs were programmed to not find this odd. Same when the program took place on Earth in the 20th century, they saw alien species like Klingons as humans.
I feel like having the capacity to question your own nature of existence sort of proves the existence to begin with. So if the fear is genuine, they too are genuine. Whether they are in a simulation is still debatable
