You are correct. The first example time in the article, "2024-12-25 at 18:54:53 UTC", corresponds to POSIX timestamp 1735152893, not 1735152686. And there have been 27 leap seconds since the 1970 epoch, not 29.

I'm also not sure where 29 came from, but the expected offset here is 27 - there have been 27 UTC leap seconds since the unix epoch.