In short, Edward Said's "main point" is that he believes Conrad to be a great writer to the point that his words can't convey their meaning appropriately as the content is too advanced.
Said openly admits his intentions at the beginning of his essay:
I hope to be able to show that both in his fictional in his autobiographical writing Conrad was trying to do something that his experience as a writer everywhere revealed to be impossible.
This means, says Edward Said, that Conrad was so advanced that he didn't really understand what he was actually saying. Edward Said considers this a grand irony in Conrad's writing. Conrad furthers this irony by comparing "meaning" vs. "seeing." Conrad uses darkness to show that even though the reader can see, the reader might not be able to understand. Edward Said is saying that this, ironically, applies to Conrad himself.
Edward Said does, however, believe in the greatness of Conrad's presentation, in fact, so much so that Edward Said believes that Conrad was "misled by [his own] language." Therefore, Edward Said believes Conrad was "misled by language" precisely because Conrad's works are so "great [in their] presentation." The gap between denotation and meaning was "widened" by his writing. This became a "problematic concern" to which Conrad came back again and again.
Edward Said talks a lot about the focus on Conrad in regard to motivation behind the stories. This means a general focus on imperialism. Conrad allows his reader to "actively comprehend how the machine [of imperialism] works." According to Edward Said, Conrad allows us to see the power of imperialism while approaching secular criticism of that imperialism.
According to Edward Said, Conrad could only achieve a "visual outline" and remain distant to real meaning. Even though the meaning remains obscured, we can still see its general silhouette.