CG - you are absolutely right - we were certainly told "if in doubt pull the reserve". What the Major meant to say was that, with the numbers in the vertical descent from the balloon as - initial height 800 feet - drop of 200 before the chute should open - time waiting for something to happen (still falling at an ever-increasing rate!) then pulling the reserve didn't leave much time for reserve to open.
For the same time delay after leaving the aircraft (with a forward throw) the height loss was much less - so more time to sort it.
Anyway, it all worked so job done