It might even be so that the younger pilot did not consider him to be PIC at all. After all, according to the report, he had said to the CFI that he was not legally allowed to fly with passengers because of the 90-day rule. The CFI then told him that he would fly with a more experienced pilot and this may well have lead him to the conclusion that the older pilot would be PIC.
This is then reinforced through the fact that the older pilot had made the decision to fill the tanks to the brim. In a -140, with four adults on board, that's not something you do without consulting the PIC. So again this may have give the impression that the older pilot would be PIC.