If you were to take a full and well crafted running program and then randomly removed 1 or 2 workouts per week from that program, you would presumably be slower. If you replaced those missing workouts with comparable swimming workouts (similar effort levels for similar amounts of time), you may find that person is slower than if they did all of the running workouts, but faster than if they had just been missing the workouts.
A small minority might believe they are faster if they do some cross training because they can't handle high mileage of running or high intensity of running, etc. For a very small percentage of people, this is probably true.