When I did mine I just used a foot pump and a few pumps .. it only needs to come out about 4-6 inches .. I "think" the reason is to give enough extension to get the thing fitted, without creating a vacuum in it by pulling it out .. if that makes sense..
My copy of Haynes says .. at Ch10 section 11 para 13
13 On models with rear suspension level control, charge the shock absorber slightly with compressed air. An exact initial pressure is not specified by the manufacturer but carefully inflate the shock absorber until it protrudes through the upper mountings. Refit the pressure line to the shock absorber valve and secure it in position with the retaining clip.
Yeah, see, that doesn't ad up IMO. I see what their saying but it takes no account of the fact that the shocks come compressed! Their strapped down with that nylon pallet wrap stuff in their lowest ride hight position, so to speak. So cutting the straps which are under tension by the shock means they extend fully under their own steam anyway.
When fitting mine I made a mental note to leave them strapped up until the bottom bolt is tightened up, then cut the strap and let them extend into the turret hole.
Only reason I can think of for pee inflating is if the bladders stick together while in the box, then possibly causing a rip in the bladder material as they extend. Other than that I can't see the reason for pre inflating personally? There's nothing to keep the air in other than the sealed air connector anyway. So pre inflating off the car won't do much.
Or have I missed something again! Just seems like the book of lies up to it's usual to me?