That's not really true. What they decided to do was build copious backwards compatibility with OS 9 into what was essentially NeXTStep and then re-skin the OS so it looked Mac-like. All the backwards compatibility stuff was essentially new, I believe, and not ports of the OS 9 internals.
You can maybe squint at that quote and say it's not really false, but it certainly doesn't get it right at the technology level.
Yeah, but you didn't have access to Steve Jobs. And Isaacson probably could have also talked to Bud Tribble, Avie Tevanian, Ali Ozer, etc.
Frankly, I'm not even sure why anyone would expect Gates to have any particular authoritative insight into OS X. Microsoft's concerns would have largely involved the effort of porting Internet Explorer and Office from Mac OS to OS X. Anything new in OS X that didn't require changes in their software could be ignored.