I think it's totally this. In my experience there is variance in how effective the top percentile of developers are at their work, but not nearly as great as the variance at the bottom. There are MANY teams out there where even the best developer on the team could hardly answer any question on leetcode, and some where everyone on the team could rip through every question like a bag of chips. People who call themselves 10X developers have generally not spent much time on the latter kind of team.