Bastardized
The Ash Treants weren't just some throwaway Race of beings. They were Demon Kings, existences on the same level as the Martial Gods in terms of power and influence.
If their techniques could be circumvented just through some lower world methods and a powerful soul talent, they would have never been so lax in letting it spread to begin with.
The Perfect Blackbody Soul wasn't the only top tier soul talent with large reserves of stamina. In fact, it wasn't even that was the easiest to cultivate in this regard.
Sure, Ryu had great soul stamina, but it was at the cost of heavy efforts and long cultivation. Given the amount of energy he had absorbed through self-created pills and his new soul cultivation method, he should have long since had a Perfect Soul at the very worst.