I have never seen anything like that recommended here. If it were a big deal, it'd be in the PSchool article.
I personally don't own an in-ground pool, so I can't say for sure. Although I have experience with them, just not in climates where it ever freezes much at all and only occasionally.
But really, if you're several inches above the pool light, you'd have to have some MEGA ice in order for it to get so thick that it'd go down as far as the pool light. Not to mention the ice moves "up" as it freezes, not down. Expanding outwardly against the walls some, but not as much as going up.
Yes, if it did get down that far, I could see it causing some problems. But we're talking about a thick, glass light, whose weakest point will be center. So you'd easily have to develop a good 4+ inches of ice to even get near the center of the light cover, up to 6 to get dead center. That doesn't even take into acct. the rise of the ice as it expands upward. Then it's have to continually freeze & thaw in order to push outward.
It seems probable taken the variables, but highly unlikely given the practicality of the situation.
All of this assumes you're measuring from the top edge of the light housing, not center and given an 8" light.
But someone else I'm sure will come along with some advice, or personal experience as to the lights and their vulnerability in this situation.