At that time it wasn't solved and it was unknown whether Chinook was at Tinsley's level. Now it's solved in the sense that a humongous computer search has proved it to be a draw, but there is still competition between humans. Chess is generally believed to be a draw too. A mathematical or computer proof of that wouldn't change things very much though. The best way to beat another human might involve playing into positions that are lost for you, if you can get into a complex enough position to confuse your opponent into a mistake.