That's correct. The Astronomical Almanac is jointly published by the US and UK, and is based on the Development Ephemeris models from the Jet Propulsion Laboratory. All three of the leading ephemerides (the other two are the Russian Institute of Applied Astronomy's Ephemerides of the Planets and the Moon and the Paris Observatory's INPOP) use a first-order general relativistic correction to Newtonian gravity. They also use a relativistic time scale; Earthly clocks change slightly as the Earth goes from perihelion to aphelion and back.
Some of the observations used to generate those ephemerides are very, very old. Thousands of years old. The ancients recorded eclipses and transits, and those old records, while scanty, are a part of what give the ephemeris models a long term basis.