the same way they know that Iceman was 70,000 years old and that the Andromeda Galaxy is 220 million light years away.
(There is only One who truly knows, and He no longer walks the earth.)
How do they know it’s that old? Because if they said any younger, it wouldn’t be an attempt to disprove God.
A bit tongue in cheek, perhaps, but ever notice how many dates that get provided that are suspiciously just a little bit past the biblical chronology? It’s probably not an organized effort, but I doubt it’s a coincidence, given that the vast majority of scientists are athiests.