[fanf2]

Right.

But tidal forces are not completely irrelevant to leap seconds.

The length of the atomic SI second was calibrated to match the previous sidereal second, which was based on the length of a year. In practical terms, the sidereal second was based on Newcomb’s Tables of the Sun, which was a model of the solar system developed at the end of the 1800s based on historical astronomical observations https://en.wikipedia.org/wiki/Newcomb%27s_Tables_of_the_Sun

So the length of the second is based on the length of the year 1900, and derived from older astronomical measurements of the rate of movement of bodies in the solar system.

Leap seconds exist because by the 1960s the length of day no longer matched Newcomb’s calculations for 1900: it was longer by 1 or 2 milliseconds.