The statement that the first step is the slow step actually means that the first step "in the reverse direction" is slower than the second step in the forward direction, so that almost all is consumed by reaction with CO and not with NO. That is, "r"−1 ≪ "r"2, so that "r"1 − "r"2 ≈ 0. But the overall rate of reaction is the rate of formation of final product (here ), so that "r" = "r"2 ≈ "r"1. That is, the overall rate is determined by the rate of the first step, and (almost) all molecules that react at the first step continue to the fast second step.