Did you even read my first post (directly before the one that begins “Follow up…”)?
I said everything you said, just in fewer words and with more accurate numbers.
Although the slowing of the earth’s rotation is important, leap seconds per se are not relevant as they are added to keep solar time in sync with an arbitrary measure which came about due to lots of international confusion circa 1958 (google ephemeris time, terrestrial dynamic time, and international atomic time if you want to partake in some of that confusion!)
What is important is the value of something called ΔT (delta T). This is a measure of the ongoing difference between a time scale based on the rotation of the earth and a time scale based on seconds of a fixed length. The discrepancy between today and the time of the gemara is about 3 hours (with an uncertainty of a good few minutes either way). This is a very significant amount of time, but we don’t take it into account when fixing our calendar (the date of rosh hashana depends on the molad of tishrei with certain postponement rules), and presumably we shouldn’t take it into account for kidush levana either. Lo nitnah torah lemalachei hashareis.