This is rather oddly worded. “…adding a leap second allows astronomical time to catch up to atomic time.” It’s the opposite - the atomic time is held back, waiting for the reality of a slow Earth.
I was on the Time Nuts mailing list ten years ago, and hoo boy, you should see how mad some of those folks get when discussing leap seconds. The big problem is that they are not predictable, so time code that works far into the future is impossible to write. Your clock has to receive a message telling when the leap second is about to take place.
