Much the same technique is used for all 500 singles of the period be it BSA, Matchless, Ariel, Norton or whatever, so if you talk to the owner of any of them and watch their technique you won't go far wrong.
In the days when Army DR's rode the BSA M20 they were taught much the same starting technique as described with emphasis on the 'long swinging kick' where the whole body weight was brought to bear on the kick start not merely the the strength of the leg.
When learnt this results in a far more effective 'kick' for no more effort.
Don't worry, it's just another motorcycle skill to learn and once learnt will never be forgotten and easily used on all big singles.
Cheers, Bob