Please explain how the source is reliable.

Lol. The cursed mantra of the historian (or reporter, or student) - or it would be, if they were doing it right!

Pre-2020 I knew a source was reliable because there was a method of verification

Nothing has changed. Abject appeal to authority, credential worship and the general belief in the mere possibility of "trustworthy" sources that never require validation are every bit as anti-intellectual and embarrassing now as they ever were before. They are the hallmarks of poor/incompetent students, typically created through conditioning by rote under the guise of education. I know because teacher told me so.

The methods of validation haven't changed much either, and the most reliable dating tools in archeology (for instance) continue to be coinage (often has the date printed on it :) and pottery, going on more than a century now.

History is tricky business, and the one thing we can be certain of is - it's doctored. My view is that the best history comes from first hand accounts, preferably from someone you know and have good reason to trust, and who was making thorough recordings at the time of the historical event (diaries, dated letters, wax cylinders!!! etc.). That is about as good as history CAN ever get, in terms of "verification".

Beyond "the most reliable" history mentioned above, you are in the world of unverified/unverifiable anecdote - typically state/nation/kingdom sanctioned (and most often penned as well) - and generally designed to inform and place implicit philosophical bounds upon the "reasonable" hopes and expectations for the future within a "society" of the consuming citizen.

History, when done properly, is largely a compilation of crimes/atrocities. However, most of the criminals and tyrants wish to be remembered/immortalized as heroes/gods/kings. One of the best ways to determine who committed the crimes, is to determine who most benefited (cui bono)... 9 times out of 10 it is those criminals and tyrants whom history remembers as kings and heroes.

My advice when dealing with unverified/unverifiable anecdote (largely, history) is to treat it as a story and to try and read between the lines. The truth can't usually be entirely buried, and evidence of those crimes still remain to be found if you are diligent and demand validation (aka being skeptical).