I would want to be very secretive if I knew stuff and had been told I would be killed if I ever said anything.
That certainly doesn't sound like a spiritual system, philosophy, or Belief System (B.S. for short) that is based on what some call the "Light" (i.e. a worldview sourced from what we generally deem to be 'good' stuff like love, empathy, compassion, inclusiveness and so on).
Any B.S. that operates on secrecy and with implied (or explicit) threats of bodily harm for sharing has more in common with a drug cartel than a path to enlightenment. I just figure that if I'm going to follow anyone else's B.S., it might as well be a more pleasant/positive/plausible B.S..
Especially if I had learned that living forever could have something to do with what I knew.
And I can say that with just as much certainty and authority as Castaneda does when he asserts otherwise, and for the same reasons (or lack thereof).