I did have one game where I didn't know the theory except a very vague recollection in the beginning. I actually thought I had blundered in that game and was trying to figure out what I'd do if my opponent made a certain move — they didn't find it, I ended up winning material in a tactic and they resigned — I was in complete shock when it came back 100% accuracy (and I definitely did not see the engine response to the move I was worried about, which was the best move).

I'm only around 1600-1700 on chess.com.

Not taking a position either way on Hans, but I have no doubt he knows far more theory than I do (and I do know some lines 20+ moves deep), and correlating with an engine is not impossible even outside of book.