But sugar is literally added to jelly as the preservative, that's why jelly exists to begin with.

For example, the very first recipe for strawberry jelly from some horrible spam recipe site:

2 cups (16 ounces) pure strawberry juice (I used 3 1/2 pounds ripe, fresh strawberries to get this amount of juice) 3 1/3 cups (666 grams) granulated sugar

It's literally more sugar than strawberry juice.