To be fair here, you said he got grumpy, many do not agree with some of the ridiculous rules set out and will have a moan about them but wouldn't breach them this guy you refer to seems to be one of these and there are many about who have the same thoughts. The drivers rule book in effect is an insurance policy and the TOC's PDP is a back covering publication.
Guessing here but you probably have some sort of TOC specific speed requirement at an AWS magnet/200m from signal, say 20mph? The over-emphasis on looking at the correct speed rather than concentrating on the red itself at the end of the platform is what i meant.
As an example, that 15mph mentioned above is exactly the sort of trouble that could catch a newer driver out. Driver assumes 15mph max whilst expecting to go onto another unit, (i'll not use THAT phrase haha) does that and drops his backside on seeing two very small red position lights almost instantly because of looking at a speedo to ensure they're only doing 15. Trying to solve one problem causes another. etc etc etc.
I totally agree with you about that back covering comment, ultimately that is what leads to the frustrations of many of us. Ultimately there shouldn't be any deviations from rule book. It should be one specific set of rules that everyone obeys with nothing else.