> clearly the vast majority of the meaning of the words is derived from the writer's intention
This is not clear at all.
Written words are just marks on a surface. Whoever made them may have intended to convey something, but they made them and walked away; they are now absent, taking their intents with them; only the marks remain, and those are not sentient or even alive - they contain no intent. There is nothing about the patterns left behind in themselves that makes them different from any other patterns the universe contains as far as the universe is concerned. If handed a set of marks on paper with no other information, you have no way of knowing for sure how they came to be. You could guess, you could be super confident, but you couldn't be /certain/.
If a reader later comes along who happens to have studied the same pattern-codes as the creator of the marks, however, seeing them will make that reader recall the associations and build up meanings in their head. These may or may not be the same meanings the writer intended to convey.
Children learning to read understand this very well - reading is /hard/, associating meaning with code is /hard/, decoding similar meanings to everyone else is /hard/, aligning the associations spoken words trigger in your head with others around you takes /study/ and /effort/, even realising that you end up with different meaning in your head when presented with some symbol to what others get, though frequent, takes deliberate effort. Reading comprehension questions in elementary school tests are there for a reason.
To a well-practiced reader, the process is natural and seamless, and feels like telepathy; it /feels like/ meaning has been transferred directly from the writer to the reader. But it is not that, and many problems arise when people forget this.
Once you have learned to seamlessly decode symbols into meaning in your head and the process is fully automatic, symbols you encounter in the world will seamlessly trigger meanings in your head regardless of their origin.
This is the human condition: we are all locked inside our own heads, and you can't take a piece of yourself and place it inside another directly. The best we can do is shout into the void and hope something similar inside the person across from you resonates; but it turns out that human shouts are not the only thing that can make those strings vibrate.
When we encounter combinations of symbols in the world that trigger complex meaning inside us, we /expect/ them to have an author who intended to convey something like that meaning, because in the entire history of human experience to date, the only other way for such things to appear in the world has been incredibly unlikely coincidence, invariably accompanied by context that makes it clear it is coincidence. (A certain proportion of social media content is, in fact, people sharing instances of these coincidences!)
However, this is an assumption that we make, and the world is rapidly changing in ways that mean it may, going forward, no longer be a valid one.
More and more, we will encounter combinations of symbols in the world that trigger complex meanings in our heads but originate from no human intent beyond "I need a combination of symbols that will trigger these meanings in the heads of those who encounter them", if even that much is explicit. We are rapidly improving the processes that produce them, and one of the ways we are improving them is removing tells. You will see the symbols, they will decode into stories in your head, and you won't know for sure if a human author was involved or not.
It is vital, going forward, that we all remember that symbols triggering satisfying meaning in our head does not automatically imply deliberate human intent for that meaning in their origin, lest we be entirely unprepared for the brave new oncoming world, just the way a chunk of the populace was unprepared for nigerian prince emails in their heyday, or are still unprepared for telephone calls from "internet tech support" right now.