It’s a bit more nuanced than that. Chess engines exist, and computers have the ability to always make the most correct move given the current board state 100% of the time, even when viable alternatives exist. That’s how they tend to catch cheaters. If you’re out of book moves and making engine optimal moves every turn, there’s a strong suspicion that you are using a computer to cheat. Even Grandmasters play sub-optimal moves, so when novices come out swinging with near-100% accuracy on their scorecard, it raises a lot of alarm bells.
In chess, doesn’t that mean he was playing well?
It’s a bit more nuanced than that. Chess engines exist, and computers have the ability to always make the most correct move given the current board state 100% of the time, even when viable alternatives exist. That’s how they tend to catch cheaters. If you’re out of book moves and making engine optimal moves every turn, there’s a strong suspicion that you are using a computer to cheat. Even Grandmasters play sub-optimal moves, so when novices come out swinging with near-100% accuracy on their scorecard, it raises a lot of alarm bells.