Doesn't a written corpus distort a spoken language?

Yes, but that's okay.

In the comments on my recent post about Authentic Language, Alan Wood asks, "What difference does it make that we largely have ‘writing patterns’ rather than ‘speech patterns’?"

I think this is neither a feature nor a bug, just a given, of both what we have, and what we are undertaking. So, when we talk about speaking and acquiring Latin, or Ancient Greek, or another historical language, the "idealised corpus of speech" which serves as the objective basis for acquiring the abstract 'language', is simply the corpus of written texts we have. There is no ongoing native-speaker community-spoken community of the ancient language as it was. And, we do know well that they way language is represented in writing always differs from how it exists in the moment of speech and conversation. If there's any doubt for you of that, listen to an audio clip of a conversation and try to transcribe it verbatim.

However, what is the language we're trying to acquire? It's the language of the written corpus. We're (or at least I) not trying to reconstruct a vernacular oral language behind the texts that we have. Any such reconstruction of some kind of "this is how it was truly spoken" involves a level of speculation and tentative reconstruction. Not that this is impossible at the micro level, but I have rather large doubts about it at a macro level.

Rather, we are being acculturated and inculcated into a fossilised representation of language, embodied in texts. There will always be an inherent conservatism, then, in 'living' Latin, or Greek, etc., because the corpus is a norming norm for all new speakers.

However, the norm should be broad. We do get conversational and colloquial elements in ancient texts. You see strong elements of conversationality, colloquiality, and the like, even when stylised, e.g. in the comedies. You get a different register of writing in the sub- and non-literary papyri. For a biblical studies student, you cannot get a good sense of style and idiom if all you've read is the New Testament. To repeat my common trope, it is like learning all your French from 20,000 leagues under the sea and then wondering why you can't accurately judge register, tone, style, idiom.

Written language is a standardised expression of spoken language, and serves as a good standard to model contemporary communicative language upon.