No one in Kitakami knew about Pecharunt prior to the epilogue, why would there be signs? And who would have written a book? Ogerpon?
No human in the Pokémon world ever knew 90% of what was in the video. Or ever will, given that the only creature that actually ever knew most of the story was Pecharunt itself.
Aside from someone maybe referencing the Never-Rotting Peach name, none of the new information is actually known by humans.
They don't have to know it to be fact, but a legend about a Couple somewhere that had a Peach Child wouldn't exactly be far-fetched or bizarre for a world as fantastic as Pokemon, especially just AS a folktale or legend without knowing Pecharunt actually existed.
Also, people were speculating about Pecharunt before release because a symbol that correlates to it shows up in the art used while being told the Loyal 3/Ogerpon story during the Teal Mask. There's a child the villagers speculated to be their trainer on the Loyalty Plaza sign (first hint falling in line with the Momotaro dynamic) running with a ball floating amongst them that looks like a Pokeball but hindsight would suggest to be Pecharunt, and during the Grandfather's recount of the TRUE story, Pecharunt can be spotted in the upper right corner of the screen. You can tell that it's meant to be a distinct entity because it pulls down to "zoom in" on the view of Oni Mountainlike Fezandipti, instead of moving upward as part of Okidogi. Pecharunt isn't referenced in particular but most of the story just refers to the group as a collective of "Greedy Pokemon" regardless. This is an in-universe detail that has no reason to include Pecharunt if the characters aren't aware it exists.
More to the point this detail is just the half about knowing in-universe Pecharunt came to Kitakami at the time the Loyal 3 would have, not the much more mystical/fantastical part about it being a Mind-Control Mochi feeder Pokemon that would have originated outside the region anyway (given the Parents are stated to have heard about the masks in a "Faraway" land, suggesting they and Pecharunt lived well removed from Kitakami). The Scarlet/Violet book is also full of borderline cryptid descriptions for the Paradoxes that no one in-universe is supposed to know truly exist either, on top of them having Pokedex entries that equate them to speculative creatures in Tabloids. If these Magazines are making up Billion Year old Jigglypuff and Ancient-Civilization machine Delibird, why is a Peach Child that much of a stretch to find mentioned in a weird magazine or a book of legends?
None of this information needs to all be known by a single entity in the manner described to have it all in-game for the player to find and connect the dots. One person in Paldea or Kalos or Poke-China or just some other region far from Kitakami recounts "hey there was this old Legend about a couple here that raised a child who looked like a Peach" in a book of fairy tales, and then someone different in Kitakami recounts the Loyal 3 having something round with them when they came to Kitakami (don't even have to know it was a Pokemon) as an innocuous detail. No one remembers it well because Pecharunt got yeeted into the woods and thus its body wasn't found with the Loyal 3 after the fight, so it's not assigned the same importance in the legend that gets inflated into a tourist attraction (to the point they keep using it even after knowing the Loyal 3 were Bandit Jerks). That puts both halves in places that are only known by people who could logically have access to them, but allows the player character to be exposed to all of them so you can piece the full story together based on in-game information, even if the Youtube video is a richer presentation.