This is a bit of a play on ideas here. The passage states that we cannot use adjacent runways when there is bad weather. This reduces the number of airplanes that are allowed to land from 60 to 30 per hour. So the reduction is not in per runway landings, but per hour landings in total.
It is very possible that the runways that are still operational have just as many landings as in good weather. The only difference being that adjacent runways are not used.
I hope this helps. Please let me know if you have any other questions.