My answer to that is, Landon has killed people and took part in infamous gun fights in relatively recent memory. Why would people in town be talking about events that happened at the earliest 19 years ago (1899), to at latest 34 years ago? Red overall didn't do much asides from assassinate a rogue governor and kill some outlaws, which asides the governor, most of what he did was a common occurrence. You could argue about the gunfighting competition, but my answer to that is, how often to people irl talk about a sports event from 19-34 years ago? Without looking it up, can you tell me who won the 1999 SuperBowl?
Also, without looking it up, can you tell me the last time a senator or governor was assassinated? How often do you hear people talk about that senator was assassinated?
The events had to have happened. The story is pretty consistent with the retellings. John was originally going to be Reds son. You have all sorts of NPCs referencing the events or him. Like uncle saying he was once called the one shot kid. Which makes sense. Uncle would have been around and heard the stories of Red and his nickname and stole it to make himself seem more important. You can find Twin rocks, which confirms that the locations are real. If Twin rocks is, I don't see why Brimstone and other places wouldn't be either.
The Icons of the west even back then, got warped and the facts was lost along the way pretty quickly. Wild bill, even when he was alive, there was outlandish stories about him. Or like when Wyatt Earp somehow was bulletproof and managed to somehow not get shot numerous times at point blank range and killed curly bill and most of his gang single handedly while wading through a river.