gawd, there's room between secondary and primary, if I was turning left I wouldn't do it from primary because it's asking for something to try to pass inside you surely, their fault doesn't overly come into it, it's about being defensive surely
your argument sounds a bit too alike to a car left hooking, 'I slowed down and signalled and the bike came from nowhere office'
with all respect
the trouble with primary on a bike is that it's good for straight ahead in traffic, not so much for turns, especially left turns when there's the potential for other bikes or even scooters and motorbikes
hand signals tend to be short, not like indicators so road position is more important
imho