Hi, I can offer my opinion on this. So here it is:
The goal of this change is to milk the NLP as much as possible. At the end, your deadlift is getting heavy but it is still kind of easy to recover from in order to hit your numbers in other lifts at next training. So this is why you go for 1 more rep and still pushing forward before intermidiate programing.
The other bonus from this change is creating a chance to know how a heavy deadlift feels like. You need to learn this because later everything feels heavy. You need this confidence of pushing heavy weights.
So when you move to intermidiate programing and start to pull heavy once a week 5RM than you know your deadlift is heavy. I have seen many people go way to early out of NLP for various reasons - mostly diet, rest between sets and not knowing how to grind reps.
That is my opinion but I'm not sure if this is exactly the reason why those to coaches recommend this.