I don't even think there is a clear definition of what the "10x" is measuring in terms of output.

Features/story points (velocity)? well.. this is a productivity metric of the team rather than individual.

Without a doubt some people can be more "productive" individuals than others, but I -do- think in the real world, it makes more sense to look at productivity on the team level rather than individual. Ex: someone is very productive at making lots of commits but obnoxious to work with or has shoddy tests/documentation that end up sucking time away from the "less productive" developers that have to clean up their mess...