Hmmm... My assumption is that the Cowled Wizards had cast "Spell Immunity" and selected the school of magic which Timestop is a part of to gain immunity. I don't know of any other way that it could be done (unless they have *really* high MR).
Not sure about the demon prince...however, I think that he's like 100% resistant to magic in AD&D. That would *definitely* make him immune...
------------------
Now where did I leave that doughnut?!