That's literally what the Charm is!
We're not interpreting this the same way.
Exalted 2 said:
This Charm lets the character ... coded, obscured, and hidden communication as if it were clear.
When I send you a message encrypted with an OTP, the message I am sending you is
not coded, obscured or hidden. It's just that the message is telling you to go look stuff up in the key, which you don't have. You can understand this perfectly, it just doesn't help you.
This charm
should allow you to read any message if the understandable plaintext content of the message was present in the message itself (eg, "You shall invade Ashina on the first day of the next month!"), and you just need to rearrange the message to get it back into a readable original state and/or find where I've buried it in an innocuous letter, obscured the characters or whatever. No matter how difficult my cipher, how diligently I bury my coded message or how clever my camouflage is, you can read it and understand it. This is fine, that's exactly what the charm does. This is not in dispute.
Thing is, if all that message says is "Look up character 24 (186, 78924, 11, 3... etc) in some other document, lol", you
still can't figure out what I'm ordering my minion to do unless you have that other document. The content of my intended communication to my minion is not present in the message you are reading, you understand exactly what it's saying, but the charm
doesn't say it can pull information that's not actually there. You get my message and can read it, but you don't have
what I meant to tell my guy. That's not in the message, that's in the other document, and the charm you are using doesn't get you that other document.
A Charm that let you automagically divine what I meant to communicate is different from a Charm that allows you to read stuff through any form of code, camouflage or obfuscation. See below scenario / thought experiment.
As I said, your assumption sucks. You can keep making it over and over but it's the frictionless spherical cow.
Especially, at a table, if a person contests an OTP with a Solar linguistics charm then their roll-off against the opponent's Intelligence + Linguistics roll should be interpreted as representing whether the opponent made a mistake sufficient for the Solar, aided by their formidable magic, to exploit.
The information need not come out of nowhere, but it also the particular failure mode need not be explicit unless you feel like narrating it. (For example, as I mentioned, the information might be leaked in the handwriting.)
Hmm.
Okay, here's a scenario. Dragon-Blooded Officer Steve and Dragon-Blooded Officer Alex pre-arrange between themselves a signal. Should Steve send Alex a note saying "Hey, you remember when we snuck out of the House of Bells to get sloshed on Dragon Spring sake? We should do that again!", Alex will understand that Steve wishes to signal him that their embezzlement scheme with their legion quartermaster is under threat. They agree to this beforehand.
Should Solar Bob the Night Caste be able to use the decoding charm we're talking about to understand what Steve is actually trying to tell Alex when he reads this memo, or does he only get the actual content of the memo? Personally, I think Solar Bob will only get the actual content of the memo, and to understand what Steve is intending to communicate to Alex, Solar Bob needs to have this other piece of information that isn't present in Steve's actual message, which the decoder charm isn't going to help him with. A different charm
could help him, but
this charm can't.
If you're running a game and this wouldn't be interesting to your players, don't throw this situation at them.