I'm guessing he meant that neither Billy nor Kelvin left OU on the best of terms. With Kelvin, it was rather nitpicky with the texts that violated the recruiting rules. Billy literally "burned out" at OU (per his words) mostly due to the fact that many of the JUCO guys he recruited to OU were no longer eligible. It seems like he and his superiors wore each other down and an exit was inevitable. Obviously, any animosity with either head coach didn't last very long.
I remember right after Billy was let go by TCU, Kelvin asked him to speak to his 2002 team right before the start of the NCAA tournament. Billy told them to not make the same mistake he did back in 1990. They went into the dance as the #1 overall seed, but being closer to home than most tournaments, there were a lot of distractions. He told them to avoid them and be serious about the tournament. Hollis took it to heart and said his girlfriend called and he wouldn't pick up. I think that whole thing was the first big step that got Billy back into the OU world.