I have no idea who invented it but I do believe that it has never been part of the default fsx.cfg file.
you're right,it has never been part of any fsx versions; RTM,Deluxe,SP1,SP2,Acceleration or Gold edition.
The DisablePreload=1 TWEAK prevent to load, not a saved flight in your docs located at:
C:\Users\user\Documents\Flight Simulator X Files, but It prevent to load a special flight located inside the main FSX folder located at:
C:\Program Files\Microsoft Games\Microsoft Flight Simulator X\Flights\other\FLTSIM.FLT.
Maybe i am wrong, but as i see it the AN-2 seems to have coding who need the default values of this flight to work from.
If its the case, its not a bug.
Regards
André