It's been my experience that the fixture screws seem to be metric because they don't grip the box threads properly. I think you could (and should) argue that using alternate screws is better than the original.
I really don't think the UL listing was affected by alternate screws. What would you do if the box had 10/24 screw holes? Use the 8/32's just because they came with the fixture...I don't think so.
