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.
https://www.oreilly.com/library/view/mac-os-x/0596003706/ch1...
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.