I suspect as a cadet you were firing the No 4...for which the magazine was not normally removed, except for cleaning and maintenance. The manual for the No 4 would have reflected that.
We didn’t do that silly over the shoulder thing with the No4 but carried out the unload drill which involved cycling the bolt back and forth a few times to ensure there were no rounds left in the magazine, carried out a visual confirmation and then closed the bolt to ease springs. We then stood up keeping the rifle down the range and followed the command ‘for inspection port arms’.
Could the manual have been changed to reflect the L1A1 drill? I suppose so, but there was no reason to. Even during the changeover from No 4 to L1A1, it’s hard to imagine any unit having mixed weapons at the firing point.
As you say, there's no reason to, and the mag was only removed for cleaning (or unloading in the dark, according to the MR1909, although I've also seen archive footage of people in Malaya loading their mag with loose ammo off their No.5 and then inserting it). Given that the action's open topped you can see into the chamber and mag at the same time when checking so can verify clear. Taking mags off unnecessarily would have resulted in them getting mixed up and/or lost with Cadets, and they're not necessarily interchangable between rifles.