Lenny was one with incredible experience when it came to Demon and devil anatomy. Of course, he could not claim that he knew it all, but he definitely had enough knowledge to offest certain things.
Firstly, Once the core was formed after the individual got to the Deep Demon realm, the core automatically becomes the center of the person's life and not the heart.
After all, the heart pumps blood, but the core was essentially the individual's power source. At the same time, it was also the connection that linked a person to the powers of the cosmos. Essentially, it was the window that granted one the favors of the cosmos, or the restriction by which one was bound.
Thinking thus far, Lenny had thought had as to how the Devil King was able to use the Poison of a Jin. Of course, he knew that the devil king consumed a Jin. But these devils were not like him.