Not saying that never happens but it sure would be the exception and not the rule. Carbon is very strong when pushed and pulled within its designed parameters but it does not take well to impacts that push or squeeze the tube like falling sideways on a blunt object. Take your top tube between your thumb and fingers and give it a squeeze. I bet you can squeeze the tube enough to deform it. Sounds like the bike took the blow just fine and it also sounds like Cervelo stood behind there replacement policy as it is stated. Believe me, I am no fan of Cervelo's warranty dept having had a problem with them before but in this case it sounds like they were proper.