I think you've introduced a new dimension that is irrelevant to the original argument -- namely the original debate in this thread fork was that "knowledge sharing" is not what is killing your job security. Your comparison to John Carmack, is more an argument about competency rather than knowledge-sharing, and so yes that's pretty much a trivially obvious and true statement: a highly competent employee has more job security than a less competent one. Great, you win that point, nobody is arguing against that.

But also, do you not see the irony in the fact that, aside from John Carmack's obviously high degree of individual competency, another huge aspect of Carmack's value as an engineer, is how he's dedicated a huge portion of his career and his time to -- wait for it -- SHARING his knowledge and his approach (and yes, his personal setup? [1]). Carmack is actually an excellent proof-point of the positive value of knowledge-sharing and building a career on a foundation of openness, rather than an argument that engineers should selfishly guard their secrets and retreat into their own personal caves to guard their perceived local/private moats.

[1] Just one random example, out of probably thousands of such interviews or blog posts or whatever that he's probably done over his career: https://www.youtube.com/watch?v=tzr7hRXcwkw