Perhaps "10X engineer" is just an easier thing to say than "5 to 20X engineer as described by this paper." Perfect numerical accuracy is not needed to make the point that there is a lot more variance in the productivity of engineers than there is with most jobs.

If someone suggests you focus on the 20% of customers who make 80% of your revenue, and you run the numbers and find a 75-25 distribution, should you call the person making the suggestion an idiot?