In some domains and markets they're probably a genuine myth.
Remember, for every Bellard, Carmack or jart: there's at least 100,000 glue engineers who are just trying to cash in some easy VC checks - can't blame them at all.
I don't know... As you imply, there's a lot of mediocrity among those 100,000+ glue engineers trying to cash-in. Being 10x faster/better/whatever than the average of that is nowhere near as challenging as being 10x compared to the average kernel contributor or systems programmer. Which makes being Bellard and Carmack even more impressive.
Remember, for every Bellard, Carmack or jart: there's at least 100,000 glue engineers who are just trying to cash in some easy VC checks - can't blame them at all.