Also, although long term, the earth is slowing down, the reason for the addition of leap seconds is the same as the reason for the addition of a day during a leap year....ie the length of a solar day is not precisely 84,600 seconds, just as the year is not precisely 365 days long.
The slowdown/ wobble is the reason that leap seconds are not added predictably (or not very predictably....they're usually agreed 6 months in advance). If you look at a graph of the earth's rotation you'll see that recently it's actually sped up a tad....that just increases the interval required between adding of leap seconds.
So Dr L was not being entirely accurate (or perhaps was simplifying for the sound-bite?).
I do have some background in this, SS, my first degree being in Astrophysics with a postgraduate degree in Geodesy. I was on the first GPS training course held in Europe, in, I think, 1984 and have been working professionally with GPS since 1985.