If that's the case, it follows - for example - that the maximum seat configuration element of the definition of "cost sharing" flight has no consequences if there is no hire or reward.
My 8 mates and I are in fact sharing the costs of the flight in the PC12 hired from Aircraft R Us PTY LTD. Because of the maximum seat configuration element, the flight is not a "cost sharing" one as defined and, therefore, the flight is a "passenger transport operation". But it's not conducted for hire or reward. Accordingly, the applicable rules will be those that would apply if the flight did satisfy the definition of "cost sharing" flight (subject to the 201.025 instrument question).
It can't be that a PPL hiring an aircraft from someone to go on a flight of itself renders that flight one conducted "for hire".
Time to solve the mystery of whether any instruments have been issued under regulation 201.025....