keithl... You would probably only need to fly level if your company SOPs tell you to do so. The point raised by DFC, subsequent to your post, is valid and relevant. The design of the procedure does not account for a false GP and the procedure designer doesn't care whether a pilot opts to intercept from above or below the GP.
The design of the procedure includes the level segment for the pilot who wants to fly level to help slow down and make any necessary configuration changes prior to GP intercept. We figure that pilots will have ways to determine whether or not they have intercepted a valid GP.
FWW... perhaps I misunderstood you intially. It would now seem that what I should have suggested was that, as your Fleet Captain has agreed with the ATC ruling, you're stuck with his directive as a SOP. That being the case, it's a bit irrelevant as to whether or not ATC's ruling is correct.