I wonder if they gave the chess bot X seconds of thinking time in an era when computers were slower? The way you set difficulty for turn based game ai is that you limit how far ahead the algorithm searches. If you set the lookahead based on compute time your difficulties will be way out of line if someone upgrades the CPU.
https://en.wikipedia.org/wiki/Turbo_button
Blizzard did a similar thing in World of Warcraft during the beta. After playing for a while, your character would get "exhausted" and start earning half experience for killing mobs. The only way to stop being exhausted would be to log off or spend a LONG time in an inn. At some point, they flipped the script. They made the "exhausted" state the default, and while offline or in an inn, you would gain a "rested" experience buffer, where you would earn double experience.
The mechanic worked exactly the same, but by giving it different terms, players felt rewarded for stepping away from the game occasionally, rather than punished for playing too long. They also marketed it as a way of giving players a way to "catch up" after spending a day or two offline.