Yeah the link is broken, I had to google it.

The age estimates are based on having found a correlation between parental age and different types of mutation. The different "letters" of DNA are chemically different and the various kinds of single-letter swap mutations happen at different rates. They found by looking at present-day babies that babies with old mothers tend, on average, to have different kinds of new mutations to babies with old fathers. They then extrapolated this idea to the database of historical mutations.

How many thousands of years ago each mutation happened was estimated by a different team in a previous paper, also using statistical methods. I don't know the details of that part but I'm guessing if a gene variant is super common and widespread, it's probably old.

This all produces estimated averages for each era - they know they can't actually put an exact age and date on any one mutation, but they don't have to to get a rough average over thousands of mutations for each millennium.

It's not based on church records, it's based on statistical modelling of a database of 45 million mutations in modern people's DNA to estimate ancient mutation rates.

So the actual bias is: the ancient people whose ages are being estimated are from around the world, but they are all mutants who have many living descendants. If someone did not have a new mutation, or if they didn't have a bunch of great-great-grandkids and pass their mutation on to modern people, they aren't included.

Pre-Columbian Americans do have living descendants in the USA so I would imagine some pre-Columbian American mutants are included.

But the mutants from 250,000 years ago are all from Africa because everyone was from Africa then.