Moment by moment throughout our lifetime, our brains hum with the work of making meaning: weaving together many thousands of threads of information into all manner of thoughts, feelings, memories, and ideas.
The human brain is like a memory system that records every thing that happens to us and makes intelligent predictions based on those experiences.
I can well imagine that certain writers, even writers that we'd consider today very great writers, may not necessarily have tested highly on IQ just because of their numerical skills, or maybe they may not be very good at memory, and are not particularly good at these kinds of tests.