He doesn't flinch. A dath ilani may not have seen illusion spells but they have at least seen movies. No movie of anything like that, of course, it would leave scars on whoever saw it.
But some things are worth getting some scars, even on your mind and core, and Keltham's god evidently thought this was that important.
"I hope you know what that -" Keltham starts, and then realizes that if he continues with this ill-advised 'speech' business he's going to vomit, so he stops his breath and clamps down on abdominal muscles instead.