Doesn't that mean that one can use a PPL holder to ferry an aircraft, with him paying nothing
Yes, one club I was in did it all the time. One well known company used to do that every weekend for years when PPLs ferried the aeroplanes between the Commercial Training base where they were used all week and the PPL schools where they were used at week ends. The benefit you refer to is irrelevant as again it has nothing to do with the carriage of passengers i.e. Public Transport
I think you are on very difficult ground here. Even if there is no carriage of passengers involved, the pilot is doing aerial work. A PPL may not fly such an aeroplane for the purpose of commercial air transport, public transport or aerial work (except in circumstances given in the ANO).
If the PPL is receiving a benefit in kind (free flying) is it aerial work??