The whole idea is that some research suggests that vitamin D may be harmful without vitamin K to offset its effects ("so that calcium doesn't build up in the vessels but goes to the bones where it's supposed to be"). That's all there is to it.
No need to discuss absorption. Furthermore, the idea "taking D and K at the exact same time reduces the absorption of D" is unsubstantiated in medical research
Let’s say it is 30%, there’s no way potential buyers, when they see ‘1000 IUs Vitamin D’ for example, will realize it actually means ‘700 IUs’.
So I don’t see how your comment makes sense.