I have used the charcoal method, it really worked well.I had a neighbor who always burned them out with charcoal briquettes. It took a couple days, but it worked. On the liquid method, I heard some old-timers talk about pouring buttermilk into the holes to increase the biological activity.
I wondered if that's what he meant, as well.OP - do you mean "stump" as in "still attached to the roots"? Or "very difficult to split round of wood"?
Seriously, I think the stump killed him. I've lost a lot of sleep about this. I just need to see how it turned out.