Can freezing break up sugar polymers?
Freezing breaks the cell structure of an apple, not individual chains of short polysaccharides you can feel the taste. What happens is probably enzymes which the apple contains in cell structures go out of these structures and split long polysaccharides (which have no taste) into short sugars which you feel as sweet.

In other words, the apple digest itself by its own enzyme systems after breaking the cell structures.