That, of course, has been debunked - it was a story written by Arthur Machen which was mistaken as an actual report. In today's internet age, with the likes of Wikipedia and Snopes out there, legends like this are harder to take hold, except amongst those already inclined to such beliefs - although there are a lot of those.
Legend doesn't have to be true to be myth. Not only that, but the word "Myth" somewhat infers that they aren't.
You realize you're making this argument during the age of the anti-vaxxer and at the peak of the Flat Earth Society, right?
So many variables! I have a couple of questions, but the answers are not for me, they're for you, so don't feel compelled to answer me here, 1) Was there an active cultural impetus to obscure the events in question? The 1950's - as you engage it and think about it today in 2020 - never happened. June and Ward Cleaver represent like 20 people during that era. The other 2.5 billion get no representation at all. It's easily one of the most actively retconned slices of modern history because of the era that came directly after it, the social revolution of the 1960's. In order to demonize and vilify the social changes that had already been in motion in the 1950's, the entire era had to be bleached to the bone and recast as Happy Days, a halcyon era of moral perfection and rectitude. That is, of course, bull-honkey. The 1960's happened because the 1950's were really shitty for huge swaths of people, and the adult women of the day had learned, during WW2, that everything they had been told about needing a man to fulfill them, provide, "bring home the bacon", was a ludicrous lie. That was only about 60 years ago if you take into account that the paradigmatic 1950's start at the end of WW2 and stretch to 1965. This is supported by the generational divide that ends Boomers in 1964 and starts Gen-X in 1965. 60 years for most of what you think you know about that era to already be well into its transformation far, far, far away from fact. 2) Is there an event that draws a hard line in the history? In Science Fiction and Fantasy, the Apocalypse Trope comes in three very basic flavors. a) The apocalypse is NOW and we are living through it. b) The apocalypse was created by our parent generation, we are the sad remnant generation picking it's way through the remains. c) The apocalypse was long, long ago and now our world is utterly different from the world that came before. The intro to Thundar the Barbarian quite literally details option C, which is the one typically deployed to keep things in our world, yet allows for a very different new history to arise. You mention that there is such an event. That would serve the same purpose, so long as it was sufficiently encompassing, and it would take maybe two generations for attrition to delete the remaining population that had firsthand knowledge of the world before the event.
There's also the issue of how reliable witnesses of the original event are... there's a saying about war stories that "the only difference between a fairy tale and a war story is that one starts 'once upon a time' and the other starts 'no shit bro , I was there'
