It's not that hard to do this, the chess engine crowd figured it out back in 2001.
They really didn't. No one likes playing against weaker chess engines. They play perfectly like a higher-rated engine and then randomly make an obvious blunder. They don't play naturally like a human player of that rating.
The weaker AIs in Civ games do a far better job at "playing to lose" than low rated chess engines. It's not even close!
Yeah, the ability of chess engines to play like a human of a given rating is a pretty recent development.
And it's still a constant complaint that Stockfish suggests moves that no human could really play and follow up correctly.