Who? Define “from scratch”?
I’d guess the new regs are different enough that not a single part was reused from the old engines. The lower combustion ratio, the removal of the MGU-H, and the increased impact of the MGU-K are HUGE changes that require a very different engine.
IIRC RBP never took over the Honda development or even the maintenance of the engines. The dev freeze kinda made that unnecessary.
Also IIRC, RBP hired some people from Honda. I doubt there were allowed to use Honda IP for their engine, but you get a lot of knowledge w the right people.
Given how late Ford came to the party with these engines, I doubt it’s much of a technical partnership. I don’t think Ford has any experience with a jet ignition engine, nor much with this level of hybrid. Prob more a branding deal for now. Maybe we’ll hear more about it later.