We used to get similar claims from F1 drivers about their carbon chassis losing stiffness after half a season of racing. Having torsion tested numerous chassis, there was never any evidence to back up those claims. It was just a mind game to rationalise defeat. Funnily enough they rarely complain when winning. I expect it’s the same with pro bike racers.

A carbon structure will basically maintain its strength indefinitely unless physically damaged. Which can of course happen with even relatively minor accidents. I’ve also seen carbon frames snap at the point where they had been clamped on the team car roof. There was a thread here a few years ago about a Ridley frame that had broken like this. It had broken while riding along about half way up the seat tube with no obvious stress point. Someone then posted a photo of the bikes clamped on roof racks in exactly the same place where it broke.