@tbeason has it right here — the “defining a separate function” is talking about defining f itself (not fdot), but boy can I see how this is confusing. It’s defining the functions as a way of demonstrating the performance of: 3x.^2 + 4x + 7x.^3 vs. @. 3x^2 + 4x + 7x^3… but then since it has f already defined, it notes that it could simply be f.(x). The clarification is simply saying that you needn’t define an f just to get this performance if you happen to have a polynomial expression like one of the above in the body of a more complicated function.
So it’s in dire need of a change, but the clarification should go the other way.
I should be clear: thank you for opening the PR! This is how we find things like this and make them better.