The reason people say it's a myth is because the study that purported to identify this concept was found to have an extremely small population and confounding factors. In addition if I remember correctly it tried to do this identification by using a contrived programming problem.

There are obviously software devs who are more productive than the average. This is true of every skill. The myth is thinking that (a) companies can somehow identify these people in advance, and (b) it is better to prioritize building a team with these supposed rock stars than it is to build a team of potentially average developers who know how to work together, and then properly manage, support & motivate them. A team of ten properly supported 1.5x programmers will beat out one 10x programmer every time. And in many cases the "I'm a 10x dev" personality type does not play well with others.

I'm a firm believer that any genuinely interested, motivated and at least mildly intelligent dev can be made highly productive by finding the right fit. It's far more important for companies to focus on fit and on ensuring that their own managers actually know how to manage than on trying to tap into a hidden stream of 10x devs.

I guess it boils down to the fact that I think many companies absolve themselves and their mgmt team of blame for poor performance by saying "well we just haven't been able to identify 10x devs yet." They expect to be able to hire a single employee who will save the day for them, rather than hiring and training good mgmt.