Also to be considered: What is "a significant
difference in performance
Bike races are typically not uphill time trials. So the difference of a few seconds or 10 yards can be the difference between "being in the lead pack" and "dropped", which typically translates into an enormous
difference in final time and placing.
So no one is claiming that a light bike makes up for poor fitness, but a light bike can contribute to a significant
difference in performance.
I would suspect that nearly everyone who actually races would have had the experience of being only one or two bike lengths behind a group at a critical point in the race, yet just be unable to close that last gap no matter how hard you try. Since you are already as fit as you are going to be at that point, I would be delighted to have 1.5 kg less bike to have to accelerate.
Of course if you don't race, and just time trial up hills, you would never notice the small difference weight makes and it would be swamped out by a lot of other variables. So it could also be said that it DOESN'T make a significant
difference to performance