Nah, I disagree, as soon as you hit anything like wet grass you can engage diff, it does get you out of extreme situations but it would be perfectly suited to the sort of ground he is trying to cross.
The problem is a lack of basic understanding regarding diff lock, I blame the stealers, I remember a stealer explaining diff lock once and made it sound like rocket science and would scare many school run mum types into thinking it would seriously damage their vehicles unless it was used in exceptional circumstances.
So Difflock - as soon as you hit any ground you might loose traction (i.e. not good dry surfaces offering good traction) bang it in, its easy to do so, you can slide it across while you go, you don't need to stop to engage it.