Jian Wushuang murmured, "I reckon this stone staff probably helped Zhong Hun to execute that magnificent soul attack. Although I know about soul attacks, it's not my forte and this stone staff is only useful for me to a certain extent."
A cultivator had limited capacity to cultivate and that applied to even super-geniuses.
Jian Wushuang was a prime example. He meticulously studied sword principles and sword techniques and was also working on sharpening Space-time Law, Transmigration Law, and several other extremely powerful secret techniques at the same time. Under those circumstances, he was unable to split his attention into researching soul power.
His soul had improved over the years with the only progress being in increasing his soul power. He did not work on mastering soul attack techniques. It did not mean that he was uninterested in pursuing the area just that he had too much on his plate.