I think you're misunderstanding what "10x developer" is supposed to mean. The impression I've gotten is that they are considered to be 10 times as productive because of the work they don't do. They write code that has less bugs, is more scalable, requires less refactoring, is easier for other developers to understand, etc. You'd never see it in LOC or sprint points completed, but there'd be a noticeable difference of pace between a project of 10x devs and a project with 1x devs.