This is a good point, and part of the unwritten rationale of the argument I was trying to make.

At first glance, knowing how to spell a word and understanding a word should be perfectly orthogonal. How could it not be? Saying that it is not so would imply that civilizations without writing would have no thought or could not communicate through words, which is preposterous.

And yet, once we start delegating our thinking, our spelling and our writing to external black boxes, our grasp on those words and our grasp of those words becomes weaker. To the point that knowing how to spell a word might become a much bigger part, relatively, of our encounter with those words, as we are doing less conceptual thinking about those words and their meaning.

And therefore, I argue that, in a not too far-fetched extremum, understanding a word and knowing how to spell a word might not be fully orthogonal.

Well, I wouldn't say they're completely orthogonal, knowing how a word is spelled can sometimes give insight into the meaning of the word. I think they're mostly orthogonal though; it's fairly common for people to know what a word means without knowing how to spell it, and on the flip side there are people, like Scrabble players, who know how to spell a lot of words which they don't really know the meaning of. I've heard of one guy who is a champion French Scrabble player who can't actually understand French.