Thanks to reader C for the heads up on this tidbit.
The script writer of Quantum Chess has added comments to the forum thread where he first posted about the project. He had this to say about Keanu:
To think that this stuff that I was coming up with at home, or at my friend’s apartment.. that Keanu would have no reservation saying it. Wow. He was so into it, so enthusiastic, so professional. It turns out Keanu is a huge chess fan (he came up with the “Shāh Māt!” at the end – that’s what would have been said in 6th century Persia when the game was in its infancy, completely playing into the idea that Keanu is ageless) AND is well studied in quantum mechanics. It was actually intimidating being in conversation with him on both topics because he actually knows more about both topics than I do.