If you manually unrolled the inner loop during the argument, would that have worked? (I’m not saying you should have had to, but sometimes you need to pick up a clue by four and whack someone with it.)

See the follow up comments about the experience in the Reddit thread. Everything bounced off this guy. He even thought he had “got me” by asking “what happens when the words are really long?” And I was like, “if the average word is long, that just means there are fewer of them for a given n and it cancels out.” I then clarified definitions about what “n” refers to and showed how it’s the same thing as “average word length L times number of words w”.

I even asked him what he would need to see to believe it was O(n) and he had no answer.

Nothing worked!