The description is correct. This is related to Buddhism with which I’m just a little familiar, but as a native Japanese, I can say there is a slight difference in nuance. When we use Muu (無), we have something “completely” indefinite, intangible, unclear in our mind. On the other hand, as the description that you have given says, Kuu represents sky, which when native Japanese people imagine it should be open sky without any cloud. And we imagine sky(Kuu) as a kind of box. The box is more definite, tangible, clear in our mind, while often it is intangible in reality. When we say I feel empty, there is two ways to say it. 1) 無(Muu)の感情だ 2) 空(Kuu)っぽの気分だ In reality, both are intangible, but there is negative nuance in first one (Muu) like “There’s nothing I can do” because it is complete intangible emptiness. But in second one, it has a more positive side like “There’s still possibly something I can do”, because (Kuu) is more tangible in mind. The reference with boxes is just a my theory, but a takeaway is that Muu is intangible and can have a negative meaning but Kuu has more tangible and has a positive side.