An expert reasons, plans ahead, thinks and reasons a little bit more before even thinking about writing code.
If you are measuring productivity by lines of code per hour then you don't understand what being a dev is.
They didn't suggest that at all, they merely suggested that the component of the expert's work that would otherwise be spent typing can be saved, while the rest of their utility comes from intense scrutiny, problem solving, decision making about what to build and why, and everything else that comes from experience and domain understanding.
Oddly enough security critical flows are likely to be one of the few exceptions because catching subtle reasoning errors that won't trip any unit tests when reviewing code that you didn't write is extremely difficult.
This is EXTREMELY false. When you write the code you [remember] it, it's fresh in your head, you [know] what it is doing and exactly what it's supposed to do. This is why debugging a codebase you didn't wrote is harder than one you wrote, if a bug happens you know exactly the spots it could be happening at and you can easily go and check them.