From the interview with Loriaux (L) it seemed the story was like this:
1. Williams (W) starts making the new car while L is busy with the racing cars
2. (some time later)
L joins the Rally1 Group and is responsible for some parts over some 6? months, but is not the main designer (W is)
3. L leaves to Hyundai where he can design all parts of the car as he wants ( probably also with better budget)
I don't see much need for speculation. L was at MSport also when 2017 car was designed and seemed to take no part in designing it.