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.
That's easily above 10^30 positions. 10^30 is a very low lower bound, too.
So, uh, no.
It's also not strictly exponential as n increases, pieces are removed from the board for example restricting the number of legal moves as the game progresses.
There's probably interesting solvable subsets up to large numbers of n, like maximum captures as the game progresses, or all games where there are zero captures or some such.
It's not a simple matter of calculating every possible theoretical legal or non-legal state of the board with all the pieces.
As the pawns are moved to make way for the back row the number of legal moves increases very quickly. For example if white plays d2-d3 then it frees the King, Queen and Bishop to move; 2 possible moves are removed and 8 are added.
(I'm sure in the general sense games with a very large n aren't as I suppose a game could be played in perpetuity)