Although Jeopardy does not repeat questions per se, they often find different ways to clue the same fact or connection.
I would guess that if you took the text of a clue, ran some trivial clustering on the J!-Archive contents, and instantly displayed the top three clues and responses for a human, they could be quite competitive on the show even if they knew nothing on their own.
So if you put the J!-Archive into a Leitner-system flashcard app and put a few hundred hours into it, I imagine it would improve a contestant's chances considerably. It doesn't look like much, but that looks like the major tool here. If I had won my last game (the final regular game of Season 23, followed by a six-week hiatus) I was planning something like this. However, I never thought of using analytic feedback with categorization in this way. That clearly also gave him some valuable feedback on what to study and what categories to be confident about.
However, this is to take nothing away from Roger Craig's exceedingly impressive untrained skill at the game, which he was able to train effectively to unreal levels. He plays Jeopardy! like a boss.
To be honest, there's little information other than what he divulged out there on the net, as it was a private tool. If I find more i'll write a followup, it's pretty fascinating.