Well, it's more like logical conclusion than anything, since there're literally boundary between any and all thing, and some of them are infinite, or at least boundless. She need to be able to comprehend them all in order to even attempt to control it. This maybe a good example:
"Merry said that if we refer to the world we humans observe, which is dominated by photons, as "the world here," then there are also countless "worlds beyond" dominated by other types of particles. And in those "worlds beyond," there are life-forms living there as well."
Merry here basically have a in-development Boundary Perception, and what she seen is already "countless". Comparing that to Yukari who have "already mastered boundary from a long time ago."...
But well, more accurately, I should said "countless" rather than infinite. Though considering that she live in a String Theory universe where concept of brane world exist (and 21 dimension...), that's alot.
Well, she does said even killing her is useless, but I have no idea if destroying her soul really have any effect whatsoever. Well, she could definitively prevent her soul from leaking out by afixing her soul to her body with motion/stillness, or even making destroying it useless as she flip it around with form/emptiness, or turn it into wave that have no definite position, so I guess more-or-less it's not going to work.
Edit: Well, wait for more respond though, as I'm not sure other will agree to what I said, but this is my honest opinion.