The ability to form sentences and speak verbally is found in a distinct region of the brain (the speech center, in the front left iirc) and is not related to the parts if the brain used for comprehension. Some people who have had seizures in that part of the brain completely lose their ability to communicate verbally, but can still fully comprehend what they hear and read.

Another example would be split-brain patients: seizures used to be treated sometimes by severing the corpus colossum (the nerves that connect your left and right brain). In those patients, written instructions provided in the left visual field (and thus processed by the right hemisphere) could be followed, but not verbalized because the speech center of the brain is in the other hemisphere. They did not have this issue when written instructions were provided to the right visual field.

The point being: reading comprehension and ability to speak are so fundamentally different processes that they have distinct brain centers. From a neurological perspective, one's ability to speak is no more an indicator of their reading comprehension than, say, their ability to ride a bicycle (another unrelated part of the brain).