Though, that does make one question the utility of having a DI present. I'm not in the industry but it seems to me that the DI should distance themselves from the direct operation of the train and provide oversight precisely so that things like stopping points don't get missed. E.g. On approach to the station they ask "What stopping point board are you looking for?" and when stopped "What do you need to check before you release the doors?"
That’s exactly what they should be doing, but inevitably during the course of a long shift DI and trainee can lapse into conversation and minds can wander.
My DI reckoned one of the hardest parts of the DI role was teaching experienced trainees who are doing everything competently by themselves.
The DI largely becomes a spare part at this point, but still has to remain focussed enough on what the trainee is doing to prevent a mistake.
I should add my comments above are purely speculation about what is quite an interesting general topic. Of course no one here knows what went on in the incident in question and, as someone else has stated above, that should be left to the internal investigation.