This article makes a number of odd claims about Project Builder. I'm not sure what passes for an "IDE" nowadays, but Project Builder did not have an editor at all: I'd think that'd be a defining feature of an IDE. The editor distributed with NeXTSTEP was the completely separate Edit.app (later known as TextEdit.app on OS X). Project Builder was just (1) a GUI wrapper for make (2) a project file manager, and (3) a GUI wrapper for gdb. That's it. Project Builder was neither connected to Objective C, nor to Interface Builder, nor to Edit.app. And while Interface Builder was revolutionary, Project Builder was prosaic and far from first of its kind. Here's NeXT's own documentation on PB:
I imagine what happened was: when Apple acquired NeXT, Apple's dev teams built an entirely new application, initially still called ProjectBuilder, as part of the Carbon development effort. This is what eventually became XCode. Or perhaps NeXT had this in their pocket but had not released it yet (this seems much less likely).
But this had little to do with Project Builder on NeXTSTEP 3.x and 4.x.
http://www.nextcomputers.org/NeXTfiles/Docs/NeXTStep/3.3/nd/...
XCode was a massive improvement over ProjectBuilder, not the least of which because it had an integrated editor. It was an actual IDE.