In what way does having every source be infallible enrich the world? If some legends are true and some are false you can be surprised. The book in the elderscrolls series are fallible because real people are fallible and that enriches the world. To enrich the world with lots of reading material they just write stuff down and leave the door open as to whether or not it's true.
If they had to implement everything they wrote down in a "in-universe book" we'd have a lot less reading material to enrich the game. The game is filled with lots of conflicting sources and books on information. On Lycanthropy itself claims that werewolves don't have to kill innocent people to survive. Yet gameplay in daggerfall where the book first appeared says otherwise.
I'm not saying every in-game book has to be 100% accurate at all times. You can have ghost stories, or romance novels, or any other genre in these games not be based on truths.
Conflicting reports on history depending on the author's point of view and bias are great. Scary stories that exist only to be scary stories are wonderful. But to have something like this be false with nothing addressing it is a mistake.
If they really wanted us to go "Welp, I guess werebears were made up after all," I have an easy quest they could have made to do so: In Falkreath, there's a man raving about a werebear he saw. He gives you the information (for a few septims if you don't pass a speech check) and you go off looking for said beast. After going to where he says and finding nothing, you go back to get some answers. His daughter or wife or barkeep or whatever listens to you and then laughs in your face about it, saying he's always drunk and telling tall tales. When you ask the guy about the werebear, he (clearly drunk off his rocker) tells you that you can find mermaids at the Eye of Mara Lake or some other location to show that he is clearly unreliable. Quest completed, you now have reason to doubt the existence of werebears, and you got a good laugh.
Misinformation is fine as long as there is something in game that addresses that misinformation, either overtly or discreetly.