Writing, at a level sufficient to get your point across and be well read, is within reach of most people. The writing is not really an intelligence test.
The gap is typically that if something is written poorly we can infer the underlying thinking that went into it wasn't well enough conceived to put out high quality text. You can understand this implicitly when you think about how you would write about a topic that you barely know about vs a topic that you are so comfortable with that the writing doesn't even seem like an effort. Ironically people often don't write about the stuff they know best because they presume it's obvious - and it is, to them. And then they write about the stuff they're trying to figure out - which is poorly understood and poorly expressed.
The gap is typically that if something is written poorly we can infer the underlying thinking that went into it wasn't well enough conceived to put out high quality text. You can understand this implicitly when you think about how you would write about a topic that you barely know about vs a topic that you are so comfortable with that the writing doesn't even seem like an effort. Ironically people often don't write about the stuff they know best because they presume it's obvious - and it is, to them. And then they write about the stuff they're trying to figure out - which is poorly understood and poorly expressed.