As Taubes often says in his lectures, carbs (or generally speaking, diet as a whole) is the primary cause, not the only cause. There could be a second (or multiple) cause where carbs interact to amplify the effect. It's also possible that these other causes do things that carbs do not, where carbs would amplify those effects, but not when these other causes are not present, making carbs appear to work differently in different individuals. An example, some guy gets drunk, another guy with hepatitis gets drunk - different effects.

Health is the foundation of athletic performance. Considering the above, it's likely that athletes are healthy a priori, but once they get sick with whatever we regular folks get, they lose their edge. Imagine that same lifter in the video I posted. He's obviously eating lots of carbs to grow and stay that fat (he used to be very lean). Now imagine he gets sick with something that interacts with carbs in a way that causes joint or ligament pain for example. With joint pain, it's simple, there's pain when we lift weights, we could lift through it though not without serious strain on endurance, this then would affect practice and ultimately competitive performance. With ligaments (there's a sensing organ in there, don't remember the name at the moment), it's a feedback mechanism that automatically reduces muscle response locally without going through the central nervous system, as a normal response to excessive tension in order to protect the ligaments and bone-ligament connection - lower strength. Indeed, lifters practice with excessive weights but with very short motion to train the ligaments. This then allows them to produce greater power and with greater control for the lifts.