Carbon fiber is a finicky material. It's extremely strong but also brittle. Given the long, but thin shape of the bar, it is not designed to withstand an impact from above or below.
In the case of the 135i, perhaps they're referring to the airbox hitting it from below. Say, if you went over a large hill/bump in the road and the engine lifted in relation to the body? I've never looked at replacing the 135i airbox, so I'm not sure if it's fixed to the engine or body...