Why doesn't chess need to do that to handle promotions?
Promotions are a different type of move than regular moves. Instead of moving a piece from one location to another, a promotion changes the last piece to move to another piece. So, it does not change the values of $origin and $dest. To check whether a promotion has happened, you can compare the value of $moved, which was set when the piece moved from one location to another, with what currently occupies the space moved to (space $dest). When they are different, a promotion has happened, and you can deploy code for testing whether it was a legal promotion.
Promotions are a different type of move than regular moves. Instead of moving a piece from one location to another, a promotion changes the last piece to move to another piece. So, it does not change the values of $origin and $dest. To check whether a promotion has happened, you can compare the value of $moved, which was set when the piece moved from one location to another, with what currently occupies the space moved to (space $dest). When they are different, a promotion has happened, and you can deploy code for testing whether it was a legal promotion.