I agree that I might not need to be able to build a human brain in software to be able to build something better, as with cars and legs.

And I agree that I might be able to build a brain in software without understanding how to do it, e.g., by copying an existing one as with ems.

That said, if I understand the principles underlying a brain well enough to build one in software (rather than just copying it), it still seems reasonable to believe that I can also build something better.