It's not outrageous - Datix is not user friendly and there are fiddly rules about different categories of harm, whether something is a reportable incident etc that a junior doctor might not be expected to know.
If DU had never filled out a Datix before (reasonable assumption), it wouldn't be unusual to fill it in with his supervisor explaining which category to fill in and where to write which details about what happened.
As an example, in a previous role I was doing a Datix every month or so because I worked in a service with lots of incidents of patient aggression. But in my current role I've not done one in over 3 years. It's generally not something that comes up that much.