<...>
I believe it’s quite the opposite. If I’m drawing-up a plan for a given project (whatever the context), all the detailed specifications will be known by me and my team. There writing of the specifications, or whitepaper in this case is something that should emanate from the team itself. A third external party is not likely to grasp all the full context of the project, as all the technicalities and specifics are matured within the team itself.
Now if the team want’s to hire a helping hand to help (not lead) them brush-up the presentation in a more “sellable” manner, or help along with the expressivity being a bit less technical and a bit more commercial, then fine. However, that help needs to be very much under the thumb of the project manager/sponsors.