The problem is mostly when the mistakes are as rare as the grammar pattern itself 
Or, more generally, I feel that mistake often happen on rarer patterns (due to the author being also not too familiar with it). That being said, that’s not an issue in general (I should have clarified; I was replying to @seanblue’s mention of obscure grammar).
Well, it’s fine to just learn it as it shows up, I guess? I’d just warn against always taking the usage in text at face value 
I don’t know about this specific case, but I’ve seen both なる and たる in series that are not set in a classical setting. Stuff like 勇者たる者 (or 王たる者, but I guess that one does sound old). I’ve even seen なる and たる being used in two different sentences back to back (I can’t remember which light novel though).
By the way, I just checked the whole book for other instances of なる, and that’s the only time it is used that way.
It is well edited! That’s why I took it as an example. I remember one mistake that was discussed on the forum (under the dropdown).