I thought some also played a warning of a voice saying 'warning turning left'. Also as it's triggered by the indicator, that has its own set of problems...
I still don't think we should be comfortable with undertaking on the left having the death penalty. Agreed, if all the conjecture is correct there wasn't much the lorry driver could do, but decent mirrors and using them isn't impossible, there's a big difference between 'not much' and 'nothing'.
I shoulder check when crossing the mouth of a junction to make sure nobody is about to left hook me, on the few times I've driven something Luton size of bigger I've been very wary of the inside of the vehicle when turning left. I guess it's possible familiarity takes away some of the fear that breeds that caution.