The linked pdf shows a freehub but refers to it as a freewheel, also. And shows that a 14mm hex wrench is used to remove the 'freewheel body'. States the freewheel body fixing bolt is not reusable and must be replaced.
I have no experience with shimano hubs like this. Sounds as if the fixing bolt has become permanently attached after 7 years.
Don't know if a hex adapter on a long socket wrench would work any better than the seatpost lever that was tried.