In 2014 a heavily handicapped Stockfish beat the 5th ranked player in the world (Nakamura) under tournament conditions despite no access to its opening or closing books and a one pawn handicap.
[0] https://www.chess.com/news/view/stockfish-outlasts-nakamura-... [1] www.iccf.com
What he has given up on is a single human beating a computer.
When he recently came out of chess retirement he didn't talk about it at all in either 2017 or 2019:
https://www.telegraph.co.uk/news/2017/07/06/king-back-chess-...
https://www.theguardian.com/sport/2019/sep/06/chess-leonard-...
There's nothing recent about him on https://en.wikipedia.org/wiki/Advanced_chess
I can't imagine a human doing anything besides making things worse or even.
Right now we have essentially two top tier engines -- traditional brute force with alpha beta pruning (stockfish), and ML (leela). Both alone are incredibly strong, but they are strongest and weakest in different types of positions. A computer chess expert, who knows what kind of positions favor stockfish and what kind favor leela, could act as a "referee" between the two engines when they disagree, and when they are unanimous, simply accept the move.
Ten years ago, a grandmaster driving a single engine could typically beat an equal strength engine. I don't think that's the case anymore.
But I think if you have someone who is an expert at computer chess -- not so much a chess grandmaster, and you gave them Leela AND SF, and let them pick which one to use when in the case of conflicts -- they would score positive against either leela or stockfish in isolation.
Larry Kaufman designed his new opening repertoire book by doing exactly this -- running Leela on 2 cores + GPU, and stockfish on 6 cores, and doing the conflict resolution with his own judgement.
The human can certainly no longer pull his own moves out of thin air, though.
It is unlikely that Chess is any different. Any superficial understanding by a human of which move is 'better' is just ignorance of the issues around evaluating a position. If you have statistical evidence that is something. 'But I think' is not evidence.
It might be entertaining to have a human involved. It isn't going to help with winning games.
I can absolute guarantee you that a human (who is an expert in computer chess, someone like Larry Kaufman) + engines will beat a single engine over the long run. With current tech and computing power, this is ONLY because we have brute force (with alpha-beta pruning) and ML engines that are at near-equal strength, and have strengths and weaknesses in different types of positions, and that those strengths and weaknesses are understandable.
If we did not have AlphaZero, I don't think the human would be able to add anything at all currently.
But Kasparov and others have given up on the idea that a human provides any unique insight into chess anymore. Computers are just better.