Anatomically modern humans have been around for 100-300 thousand years (definitions vary and evidence is scarce and ambiguous). Our bipedal (i.e. hands free to do dexterous things) ancestors go back ~4 million more. If acquiring extra fingers was a net advantage we'd all have them already.
> If this principle were true in general, it would imply that evolution didn't exist.
But it's not true in general, and I gave a specific context: millions of years of human ancestors using their hands to survive in a wide variety of environments and occasionally being born with extra fingers (~0.2% in modern humans, so probably a huge number of opportunities). We have five fingers, so it is unlikely that our six-fingered ancestors had a significant net advantage in general.
I remember reading an article about a present day man with 12 fingers and 12 toes who believes he has an advantage climbing coconut palms. If needs like that had been providing consistent evolutionary pressure on our ancestors, things might be different. Edit: here's a version of it: https://www.cbsnews.com/news/man-with-12-fingers-12-toes-cal...
