1 coulomb is 1 ampere per second and 1 ampere is 1 coulomb per second therefore they are always equivalent. They would only be different if you changed the time but the units themselves are based only on the current/charge in 1 second. So how come their SI definition is not the same?
You multiply current and time to get charge. So, a coulomb is equal to one amp-second, not one amp per second, which would be division. It's the same as multiplying speed and time to get distance. Speed is km/hr, distance is (km/hr)*(hr), speed times time, which is distance.
1 Ampere is the "flow" of 1 Qoulomb of charge per second, or 6.25 x 10^18 electrons per second. Now charge a 1 Farad capacitor to a potential of 1 Volt. The capacitor now holds 6.25 x 10^18 electrons, or 1 Qoulomb of charge. But they are not "flowing" (no per second component = zero amperes).