Deep Blue beating Kasparov in chess showcased Deep Blue's capabilities; Deep Blue beating Bryan Caplan would not have showcased these capabilities, despite the capabilities being there.
The analogous hypothetical situation would be a Not-so-deep Blue that fails to beat not just Kasparov, but any random chess master with a nonzero Elo score, but manages to beat Bryan Caplan. After his defeat, Caplan would write an article claiming that he's "known to be very good at chess". This would definitely say more about Caplan's chess skill than the chess engine's capabilities. And none of this would imply that chess engines are in any way impossible or unlikely, or that the capabilities are not impressive. Indeed, a chess engine that beat Caplan would have been impressive in 1975, even though we were still very far from Deep Blue or Stockfish at that point.