The reason they suggest spray malt is to add some unfermentable sugars to the beer at the priming stage.So the beer is a tad sweeter and maybe even a bit more malty.
If you use equal amounts of brewing sugar aka Dextrose you will end up with a higher carbonation, due to it being so fermentable.
Just as an example, 25.4g of dried malt extract or spray malt will give you the same level of carbonation as roughly 19 grams of Dextrose.