A 40% reduction of an ingredient doesn't mean that somehow there's a void in anyone's product (they could reduce the product size also, but that's a separate matter). It just means that the other ingredients together will represent a higher proportion of the final product in relation to the reduced ingredient.

Think of it this way: 1% milk has some of the milk fat removed/reduced, but that doesn't mean any other ingredient has to replace what is taken out (again, some may choose to add other things, but that's a separate choice), and your milk carton doesn't have to get any smaller. It just means your same milk carton has a higher percentage of the other components of milk.

To be clear, the article states: "this can be done without affecting the taste... Its scientists altered the structure of sugar so that it dissolves more quickly. This fools the taste buds, with the effect of raising the sweetness."

The way I read that, it seems to support the idea that nothing else has to replace the reduced sugar. But practically speaking, yes - if the other ingredients are costlier than the reduced sugar (as a higher percentage of the final product), then there's a good chance some other fillers could be added to maintain product weight and size.