This 10x thing is quite frankly stupid.

The distribution of developer skill (if that's even something that can be plotted) is not bimodal with good developers and bad developers, it's normal. Sure someone 2 SDs above the mean might be 10x more productive than someone 2 SDs below the mean, but the same can be said for literally anything that requires skill.

While we're at it, why don't we call Gordon Ramsay a 10x chef? Or Yo-Yo Ma a 10x cellist. While this is technically true, it's not particularly insightful, and certainly not something you should use to label yourself.

To answer your question, yes someone skilled at software development will be slowed down if dealing with a legacy codebase.