Author took creative liberties in building a “vaguely European world”. That’s it.
These stories aren’t set in a particular time period and sometimes concepts/items similar to our modern world are pulled in for the sake of convenience. Taking for example similar stories with vaguely European settings might also have cellphones, or have moral standards that are closer to the modern world than the super vague time period it’s based on.
Take an example in another fantasy world based on our current one, Avatar the Last air bender. There’s a lot of similarities to our current world/culture but there’s still a lot of deliberation taken by the creators. They’re not meant to be a one-on-one replica of our current world. In other words, they’re not “historically accurate”.
The same applies here.
Logically, there’s nothing that proves that birth controls should exist. But nothing proves birth controls can’t exist. There’s no indication that there wasn’t an earlier access to an oral contraceptive in this alternate world with an alternative history. The difference being that in our world attempts were made but remained unsuccessful, but in theirs it was successful. Which isn’t a stretch considering the presence of magic might greatly alter history. But ofc, nothing in the story says that it did happen either. The only thing we have is something that might feel jarring because the existence of birth control pills might not align with our expectations of a vaguely European society. But in the end, it can’t be proven nor disproven. It’s only loosely based, after all.
It’s possible to critique it for weak or unclear world building. But saying it’s historically inaccurate is like saying vampires are scientifically inaccurate. Weak worldbuilding and historical inaccuracy are not the same thing. And either way, the story is character-focussed and not world-building focussed.