Well from what I understood he tested just life regen, so no losim / laek and other stuff, but for the exact methodology you'll have to ask him.
I don't know if Yshari Baal has a healing spell (the docs don't say anything about that) but seeing how nightmare doesn't stop a player's natural regen it would make sense if it also didn't stop a boss regen. Again, for exact info you'll have to contact Malice.
We were discussing possible ways to defeat Yshari Baal with a caster since they can't apply open wounds, and Malice said that nightmare and void archon reanimate don't work, which prompted J to test his claims.