During a significant part of history, text was definitely considered a source of truth, at least to the extent a lot of people see video now. A fancy recommendation letter from a noble would get you far. It makes sense because if you forge it, that means you had to invest significant amount of effort and therefore you had to plan the deception. It's a different kind of behavior than just lying on a whim.
But even then, as nowadays, people didn't trust the medium absolutely. The possibility of forgery was real, as it has been with the video, even before generative AI.
To back up this claim, when fictional novels first became a literary format in the Western world, there was immense consternation about the fact that un-true things were being said in text. It actually took a while for authors to start writing in anything besides formats that mimicked non-fictional writing (letters, diary entries, etc.).
But even then, as nowadays, people didn't trust the medium absolutely. The possibility of forgery was real, as it has been with the video, even before generative AI.