Very good analysis, and extrapolation. You mentioned meaning, and also obvious simplifying patterns embedded in the rote memories, which he found harder than we do to notice, extract and use. Images, unanalysed sensory wholes, were fine, but abstract features and patterns embedded in them were not noticed. Yet abstraction and generalisation depend on being able to ignore the concrete, sensory details and focus on a few salient features, the ones that matter. First was the nature of his experiences - his strong synesthesia.
Language uses abstractions.
Vivid imagery is part of a limitless concrete rote memory, but abstraction is at odds with it, requiring you to forget or ignore most of the concrete and to abstract from it only selected bits. So abstracting is at odds with concrete imaging and remembering. And verbalising is really a string of abstractions. Third was whether his memory was an advantage or disadvantage.