This isn't meant to be harsh or cynical, but wasn't Ben Rome "95%" (or something) close to finishing the original back in '13 or '14?
This idea got into the water supply awhile ago, and it's inaccurate.
Ben played a role in developing the book, along with other devs at the time. But it was Phil Lee who wrote the main history section off of the outline that group developed, along with other writers who contributed sidebars. It's Phil who is now the developer of the book, and his draft--
his draft--and those sidebars remain largely intact.
Remember too that "mostly written" does not mean "mostly finished." All the steps I mentioned above--editing, art, layout, dev review, proofing, production--had not taken place. And they're not some afterthoughts to the writing that can be whipped out in a week or two.
The book that was going to be IlClan has - as you said - been split into two and substantially expanded and rewritten, according to Brent and other CGL reps.
Parts have been expanded and re-written, but much of what Phil originally wrote survives. It's what comes out of the book that required some tweaking. It's not as if the book was wholesale deleted and re-written. But new management had different ideas of where things should head after this book, which required different setup.
Before you ask: I don't know why the book was not moved along through the production process a few years ago. But it's moving now, and we're working at our best speed to get it out. I literally stopped editing a section I was working on so I could type this post.