I have read the same thing about Greece's Ikaria island centenarians.
As an anecdote my grandfather born in remote mountain village in mainland Greece in the 1900 was actually registered by his father having been born in 1906 as to avoid being drafted as long as possible to fight in the multiple wars fought at the time.
So birth records and certificates from that time are not really trustworthy.
I knew someone who came over with "the boat people" from Vietnam when he was six or seven; but his parents adjusted his birthday so he would start kindergarten instead - and he still didn't know for sure his actual birthday day.
As an anecdote my grandfather born in remote mountain village in mainland Greece in the 1900 was actually registered by his father having been born in 1906 as to avoid being drafted as long as possible to fight in the multiple wars fought at the time. So birth records and certificates from that time are not really trustworthy.