10x engineers most certainly exist, I've had the fortune of being able to work with a few and their true impact isn't in the code they themselves write but in the higher-level strategic/architectural decisions they make that impact the team as a whole. That being said it is also my experience that most self-described 10x engineers are anything but, so if one bases their impression of 10x engineers off those self-promoting cases then I can see how it would appear to be a myth