Ford owns a half-billion
As you point out, Ford owns a half-billion dollars worth of Rivian. Maybe the reason that the Aviator is a PHEV and not a BEV isn't much of a mystery if you really think about it... April 2019; "Rivian today announced an equity investment of $500 million from Ford Motor Company. In addition to the investment, the companies have agreed to work together to develop an all-new, next-generation battery electric vehicle for Ford’s growing EV portfolio using Rivian’s skateboard platform."