According to this article, if I understand it correctly, there was a bunch of ILM guys that visited the archives and saw the 5-footer, although incorrectly referenced to Empire:
http://www.wired.com/2015/11/wired-star-wars-cover-millennium-falcon/
“The big change on the
Falcon is that radar dish, because it was knocked off in
Return of the Jedi through some of Lando’s close-quarters flying,” Alzmann says (as if you didn’t know). “As far as the rest of the
Falcon goes, we had a whole crew that went up to the archives and saw the original five-foot model from
Empire. So the textures are from the original, pretty much. I’m sure our artists put a lot of love into it and fixed anything they found troublesome.”
I'm assuming they would have scanned it using some method. But like any other scan, the mesh will need to be fixed so they "fixed" a lot of the kitbash parts. My guess is it's impossible to do all the fixing in a tight schedule and is the reason for the made-up greeblies on the sidewalls. The basic 5-footer shape will have been retained because that's the bulk of the Falcon and is the most important and there's nothing much to fix in that department.
In the Force Awakens Cross-sections book it's mentioned that they used the ILM digital asset. There is one part on the Bandai engine deck that I thought Bandai wasn't able to model correclty -- the F1 McLaren M23 part . When you look at the cross-section from the book -- it's shaped the same way. What are the odds they could make the same mistake? This leads me to believe that Bandai did have access to the same digital asset.
We know Bandai took some shortcuts on the sidewalls -- heck, they copy pasted a good section four times (below) but we forgive them because it's still a fantastic model
. Astro might be right in that they used an incomplete model but at least the top view must have been completed then.