I disagree. They kept just about all of NeXTSTEP, apart from the UI (but keeping browsers in the Finder, etc) and Display Postscript.

They kept the Mach stuff. They kept the Unix stuff. They kept Terminal.app so you could access the Unix stuff. They kept the Cocoa frameworks, Project Builder, and Interface Builder. They retained applications as file packages. They retained file extensions in favor of app/creator codes. They replaced interpreted DPS with Quartz. They changed the menu bar from floating tear-offs to a single menubar. They got rid of the Shelf on Finder windows.

What did they not use from NeXTSTEP/OpenStep that was of any significance? About all I can think of is YellowBox on Windows.

Where exactly does he say that they didn't keep that stuff?

"There’s some truth to that, because Apple decided not to leap into a completely new system but instead to evolve the existing one."

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.

I think it comes down to, which matters? I wrote code for OS 9 (via PowerMops, of all things), and for OS X (I shipped two major Cocoa apps), so I'll vouch for the fact that you are technically 100% correct. On the other hand, what became OS X had a radically different design than Rhapsody: even comparing OS X 10.0 to OS X 1.2, the former could run apps natively that could also run on OS 9, the former could read HFS+ filesystems, the former had apps like QuickTime, the former ran Classic apps that couldn't legitimately run natively right alongside the newer Cocoa/Carbon apps, the former had legacy APIs like OpenTransport that the latter lacked. You can argue, correctly, that, technically, OS X is more NeXTSTEP than Mac OS. But I'll give Isaacson some space. His misunderstanding is well within the bounds of what I experienced within the non-Apple programming community at the time. I'd cut him some slack.

"His misunderstanding is well within the bounds of what I experienced within the non-Apple programming community at the time."

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.

Does he, though? He's sort of saying that they evolved the existing system instead of jumping to a new one, but AFAIK OS X was NeXT with a compatibility layer, prettier GUI and some other changes. Saying that anyone upgrading from OS 9 would not see a whole new interface seems wrong to me.

I believe the confusion arises from the use of the term "existing one". That could be construed as referring to NeXT, so it would mean they decided to evolve NeXT by making changes to it over many years, instead of switching to it at once.

If you read "existing one" as Mac OS 9, then that sentence would mean a completely different thing.

Well, the text talks about NeXT first, and then suddenly talks about MacOS. When he says that Apple used _some_ of NeXTs software and goes on to talk about the kernel, it seems like he gets it wrong and doesn't really understand what OS X was.

If he had, he would have realized its significance. For instance, I can't recall that he mentions how iOS relates to OSX, and the advantages this brings to Apple. He never mentions that NeXT was compatible with PowerPC and Intel and that Apple would have a working, parallel release running on Intel hardware internally.