An important distinction here is that checkers is also only weakly solved (Connect Four, by contrast, is strongly solved). https://en.wikipedia.org/wiki/Solved_game
This means that Chinook can play perfectly from the start position against any set of opposing moves, but if you play a move on Chinook's side that it wouldn't play for itself, the resulting position is probably not solved by Chinook.
An arbitrary position is in fact very unlikely to have been solved.
