I think the 10x label is just something that is easy to say and comprehend. But the real impact of a high-performing developer is probably way more than 10x. It's not really about lines of code. It might be that this person makes a breakthrough which changes your industry. It might be the difference between staying in business or not.

I've had the pleasure of working with very high-performing developers and the positive impact they can have on a team is amazing. I can see how some people don't believe it if they have never worked on a team where one person amazed and inspired them.