I'm assuming we're talking about output measured in the real world. At the forth standard deviation you might be 1 in a million, but you're still not outperforming an average person by 10X.

Then your definition of 10x doesn't apply to a software engineer either. What measurable metric does a "10x developer" outperform the average developer in?

The interpretation that I like is that "10x productivity" is more about what someone doesn't do than what they do do. It's in the sense that they seem more productive, partly because they're really good at what they do, but mostly because none of what they do gets thrown away. They're good at the intangible stuff around the tangible job. Someone can be extremely productive, but they won't be "10x" if they produce a lot of stuff that ends up being useless for any one of dozens of reasons unconnected to the actual job itself.