> their testing procedures had proven that he was not intelligent enough to do anything else
I see the final sentence as entirely detached from the rest of the story.
My take: Their testing procedures had detected he had lacked basic awareness of the situation. When he received a (written!) communication, he wasn't able to imagine that the author was not the smarter version of himself despite a multitude of clues. The idea didn't occur to him.
> their testing procedures had proven that he was not intelligent enough to do anything else
I see the final sentence as entirely detached from the rest of the story.
My take: Their testing procedures had detected he had lacked basic awareness of the situation. When he received a (written!) communication, he wasn't able to imagine that the author was not the smarter version of himself despite a multitude of clues. The idea didn't occur to him.