You may have just been lucky. I reloaded my save with Damian almost defeated twice, and entered the battlefield; neither time did the merchants have any holy water. There may be a check, though, and they offer it if you had not gotten any before, or something.
I didn't see any holy water when I entered the BF to sell loot or look for equipment, etc (don't recall how far I was when the patch added the chance to buy holy water; it was either act 2 or 3). Near the end of act 4 I decided to reload repeatedly to check for holy water. I found a couple equipment upgrades, but didn't have much luck with holy water. After my watched beep I realized I had been trying for an hour (some of which was equipment sorting, etc), so figured I was going to reload one last time and quit, which was when I got the holy water.