To be specific, in a written comment on a public forum, it would be "libelous" and not "slanderous". But to be either, it would also have to be false, and at

To be specific, in a written comment on a public forum, it would be "libelous" and not "slanderous". But to be either, it would also have to be false, and at least in the US, known to be false to the author at the time it was written or written with a reckless disregard for the truth. He says he's analyzed the published games, and found that Stockfish finds obvious errors in the way that Stockfish was said to be playing. Unless he's consciously lying about this (I'd guess unlikely?) it's not libel. And if it's true that a correctly configured Stockfish doesn't play the way it was said to play, this would be a strong indication that either intentionally or accidentally, it was not a fair playing field. Anyway, if your point was that it might be more productive to be more polite, sure! But one might say the same about accusing someone of slander.