Maybe he is correct and had a problem with his disk brakes.
Perhaps he overheated the rotors by too much, continuous braking.
Perhaps the brake pads were fairly worn or a bit contaminated which resulted in overheating and loss of braking power. We've seen videos showing the pro mechanics washing, etc. team bikes...they are not the most careful because of how many they have to prep, only so many mechanics to do the work, etc.
Perhaps he was just too hard on the braking and plain old overheated them...could have happened with rim brakes...they too would have failed when needed most.
You would have to remove the parts and look at them closely to see if there is a mechanical reason for failure.
I'd want to talk to the rider to hear how, when, etc. they failed.
It is interesting.
Oh it is a lot of fun to be in a race and the rider in front of you rolls his sewup...you know the next few seconds are going to be hair raising fun for those behind.