A peak core formation expert had a life expectancy of 4,500 years and only if you had a gold core or purple core would you have one that was above that. A gold core would give 5,000 years and that number would increase depending on the type of gold core since quasi purple cores gave 6,000 years while those with a purple core had 7,000 years.
The fact that the living witness was from 5,000 years ago either meant he was a palace realm cultivator or at the very least was someone with a gold or purple core at the peak of core formation realm.
Regardless of which it was, it meant they were people of means, who should have been able to come forward much earlier in the past thousand of years since the Order's founding.
"While I did say he is a living witness, I used that term closely. It's more apt to call that someone a proxy or secret keeper.