It seems that QKDNY benefited ZWJ specifically because he happened to have a huge internal base but not much technique or method to utilize it.
All other Greats have their own methods to utilize their inner strength to the extreme, and that's what makes them Greats. XL18Z on a fundamental level is just using your inner strength more efficient than any other palm technique and blowing through with raw power (due to your superior usage of internal strength).
So my question is if someone like Guo Jing were to learn it, would he necessarily benefit much from it? He already has unlocked his inner strength so to speak, and clearly can deliver it extremely effectively through 9 Yin and XL18Z. Xiao Feng has shown us you don't need no QKDNY or Shifting Stars to redirect forces and merge them. Considering the other Greats are likely not much worse than Guo Jing/Xiao Feng, they likely also have extremely efficient methods to use their energy though it is not explicitly stated.
QKDNY is also stated to be an inferior kungfu and not on par with the greatest of the Central Plains techniques.