If you buy an apple that isn't the apple of truth, the seer says that you failed your quest, but the game doesn't end. If that truly dooms you, the game should end. So what do you do?

Sierra games (especially the earlier ones) are infamous for letting you keep playing after the game becomes unwinnable. This is one example of that.

All you can do is restore an earlier saved game from before you made the game unwinnable.