Damage death?

[Executor]

Soul slay is supposed to be damage death, but it isn't it is 999.
This I found out by trying to slay a Sphinx with 1000+ health that couldn't be killed differently with body etherial and luck and 30+ protection.
So after about 20-30 failed attempts one lucky soul slay strikes but he's still alive?
Would this mean that it can't be mind hunted since slay can't kill in first shot?

Feel free to share interesting situations like this one.