“So, the native survivors of that eruption had a Christian calendar and kept records of the event?”
No, but Western scientists do, and they have techniques like carbon dating to estimate when last eruptions occurred. It’s likely that a lazy journalist used an exact year, when the experts probably gave an approximation.
Truly, when we talk/write of such momentous events, it really shows how insignificant we as people are.
Most of what we observe in nature is totally out of scale as to what we can control.
A Yellowstone event would be spectacular and catastrophic. And uncontrollable.