> The 10x engineer is more likely than not the person most familiar with the project and the codebase,

Nope.

To whatever extent they actually exist, the 10x engineer is the one who starts without a codebase, and creates one that is elegant, flexible, extensible and performant, avoiding mistakes and working faster than most of their peers would. They build a platform for their successors to work with that enables more than it impedes.

If you've never seen this, then fine, but that's most likely because you've not been around during the prehistory phase of a given codebase.