But you are mixing input and output. It's obvious if you think about learning a foreign language. Parsing written text silently is a completely different task

But you are mixing input and output. It's obvious if you think about learning a foreign language. Parsing written text silently is a completely different task than producing speech with the right intonations, even if you recognize the words. Reading aloud fluidly requires a certain speed and anticipation. You have to be reading the next word in your head while you are still pronouncing the previous one. This isn't necessarily correlated with reading comprehension which is purely input.