Zinn has written a few times about the reduced friction on bigger chainrings and sprockets because the chain links rotate on the chain pins by a smaller angle compared to the friction on smaller sprockets and chainrings. And when you are on a bigger chainring, there is less tension on the chain. Here is an example of some of his comments.
https://www.velonews.com/gear/gear-i...x-drivetrains/
So, maybe the coaches tell them to avoid smaller chainrings? Or, maybe in the heat of competition, that is the last thing he thought about.
Many of us tour on 9 speed chains, I tour on 8 speed chains. I suspect that the wider 8 or 9 speed chains do not appreciate cross chaining very much but the pros are using a lot more gears than 8 or 9 speed cassettes and have narrower chains. I would expect those narrower chains to withstand cross chaining better than the wider chains. And like you say, they probably don't care about chain life like the rest of us do.
One of the reasons that I prefer bar end shifters for my derailleur touring bikes is that when I feel my hand on the lever, I get immediate feedback that tells me if my rear derailleur is on the small sprockets or big sprockets, and that helps me avoid cross chaining.
My rando bike has a brifter for the rear, my road bike has brifters for both front and rear. I get no feedback from the shifter to warn me if I am cross chained or not. And I find I often cross chain on those bikes because I am not specifically thinking about avoiding it.