A 2nd edition of Peopleware summarises it; the 10x programmer is not a myth, but it's comparing the best to the worst; NOT best to median. It's also not about programming specifically; it's simply a common distribution in many metrics of performance.

The rule of thumb Peopleware states is that you can rely on the best outperforming the worst by a factor of 10, and you can rely on the best outperforming the median by a factor of 2.5. This of course indicates that a median developer, middle of the pack, is a 4x developer. Obviously, this is a statistical rule, and if you've got a tiny sample size or some kind of singular outlier or other such; well, we're all adults and we understand how statistics and distributions work.

Peopleware uses Boehn (1981), Sackman (1968), Augustine (1979) and Lawrence (1981) as its sources. [ "Peopleware", DeMarco and Lister, 1987, p45 ]