That rule's related to what he's talking about, but doesn't specifically address his concern. He was arguing that in choosing models that have died you should need to maintain coherency as well (if you can).
The rule you brought up does make it likely that, if you want to be able to move that unit later, you won't leave a gap that is too big for your models to make up for in their move (though that would be quite a big gap, even for 4" movement units).
Ed: there/their