Firstly Perez never says this in the article. Secondly not only people who are at red bull but even people who have left red bull have confirmed that the car is never built for a particular driver. Instead, RB develops the car in a way that makes it faster and that just happens to be a oversteery car. Max is able to live with it and hence is able to extract the performance. If RB had 2 Sergio's they would have needed to make a fundamentally slower car because Perez would be able to extract the max from that car and not to able to do the same with a oversteery car. In that scenario, the car would seem slower than it actually is due to Perez's limitation. Kind of like a CPU bottleneck (Perez) masking the true performance of the GPU(car)