This is somewhat circular. There isn't anyone who knows everything about the visual system. Thus, we're hypothesizing that knowing everything about the visual system is insufficient to understand what red looks like... prove that knowing everything about the visual system is insufficient to understand what red looks like.

Even given this, the obvious solution seems to be that "What red looks like" is a fact about Mary's brain. She needn't have seen red light to see red; properly stimulating some neurons would result in the same effect. That the experience is itself a data point that cannot be explained through other means seems obvious. One could not experience a taste by reading about it.

Maybe the best analogy is to data translation. You can have a DVD. You could memorize (let's pretend) every zero and every one in that DVD. But if you don't have a DVD player, you can never watch it. The human brain does not appear to be able to translate zeroes and ones into a visual experience. Similarly, people can't know what sex feels like for the opposite sex; you simply don't have the equipment.

DVD players do not require magic to work, why should the brain?