It's not that the 10x engineer is a myth - it's a fallacy. You'll find that people in the same profession (any profession) will have variance in their abilities and their return to a project, but it's a big leap to assume that someone that appears to perform much better than their peers is actually doing so in any objective sense.
The 10x engineer is more likely than not the person most familiar with the project and the codebase, knows the real reason why Mark didn't just put everything on Kubernetes or why Tanya didn't want to switch from React to Vue before leaving the team.
Or.. it's an engineer that has the ear and the faith of higher-ups in the organisation to deliver results because they're good at convincingly taking credit for them.
Generally I find that 10x engineer on here means "knows more than most about [tech stack X]" and can work quickly. That's cool if all other things are equal, but I've had to clean up more than a couple of catastrophic messes left in the wake of people that worked quickly.
Just to pre-empt someone telling me I must never have met a real 10x engineer or some other such dismissive response - I'm an average fish in an average pond. I've had people tell me how good I am in the past, both reports and managers, and have in both cases corrected them politely that I've just had more time with the problem/project/codebase than others have.
Anytime I hear someone talk about 10X engineers, let alone describing themselves as that, all it makes me think is that there's a failure of humility.
None of this belittles Fabrice Bellard. Or John Carmack. Or Chris Lattner. Or [known and respected programmer]. Their achievements speak for themselves.
You could give names of people in the field all day. There's a lot of us around. How many of the self-described or colleague-described "10X engineers" floating around are Linus Torvalds?