Even if the wizards had hermetically sealed themselves off, this is very implausible. Such glimpses of fairy tales as we get from pre-modern times are recognizably fairy tales, however they have been altered by rewriting to other genres. Not to mention that, while the great wizardly withdrawal was prior to the Brothers Grimm, the fairy tale dispersal shows the recognizably same tale all over the place. If they spread, they did before the seal-off, and if they spontaneously originate in all sorts of places, they would do it among wizards too, who aren't that different.
(No, magic doesn't make that much difference. There are certainly fairy tales that do not feature magic at all, so finding it a reality would not mean rejecting tales for that reason.)
Not to mention the Muggleborn. Certainly young halfbloods would be regaled at the family fire with the tales their parent grew up with.
