I said most. Having worked on carbon apps, I can say that substantial portions went through with no changes. You could add menus and item to iTunes with AppendMenu() and InsertMenu() from visualization plugins until they rewrote it in Cocoa roughly 5 years ago.

Obviously they dumped a lot of the OS level APIs for stuff like Control Panels and Desk Accessories, but the standard app APIs were largely unchanged.

There's no fundamental reason they couldn't have made their own OS with the same Carbon APIs that made it into Mac OS X (not that I think they should have—the NeXTStep stuff was much better).