I'd suspect that his written messages would also be coherent, if his spoken messages are. There is enormous skill - actually beyond my skillset - so be able to improvise a well-structured oral utterance of more than a few sentences. Even a few sentences requires a clear idea already in your head (before you start out on the first few words) of what idea/information you want to convey, and then good enough working memory and control of vocabulary and grammar to be able to structure an utterance that gets that across to the addressee. It's why even very clever people generally have to write out talks/speeches. I genuinely am not sure if I've ever witnessed a fluent and persuasive oral utterance that's been created from scratch on the spur of the moment. Sure, I've heard impressive speeches, but I think they've all been written and rehearsed/memorised beforehand.
Once you get beyond something of more than a few sentences and vaguely mundane subjects, the challenge on the human brain is just immense. It's so much easier in writing, as you can read back over what you've written, and see where your grammar/syntax has failed to convey what you intended to.
And even if you have got a such a brilliant brain that you can do that orally, it's still usually easier to comprehend quickly and accurately when it's written down, as the reader is having to reverse the process, and the same challenges to the human brain are present (working memory, etc.).
