Scientific studies are really interesting because sometimes they test commonly held beliefs, doing things because that's the way they are always done, doesn't mean it's the most effective way. There are some really interesting things in the paper, for example their is quite a dramatic change just by adding a nesting box, I like that they compare things to wild rabbits too. The triggers for the kits leaving the nest are interesting as well.
However you also have to look at context, for example if they are rearing in a more confined area, where there is reduced enrichment the kits may be nursing more than is healthy for them or their mother. Whilst to us, from a pet owner point of view, the obvious thing is to give them more space, this paper is looking at improving welfare in breeding across a much wider context of breeders and probably countries. It sounds like in some of those situation the welfare of mother and kits is improved by removing the kits at 4 weeks. However, I expect although removed from the mother, they'll still be keep as a group, in similar housing/conditions and the same diet - which would address many of the issues that we'd associate with removing rabbits at that age. It's not quite the same as spiting them up, transporting them to shops, selling them on to a new home - three changes in situation/diet etc.