Probably shitty grape/apple liquor that they sweetened up with some maple syrup to make more palatable. You can brew maple sap beer, but I'm pretty sure a straight distilled liquor from the syrup is impossible. You could use it in place of white sugar in a mash and then distill that, but you would still need corn, potatoes, fruit whatever as a base and unless you're sitting on a large amount of maple syrup that you got for free it really wouldn't make any sense. Even when making the sap beer you still pasteurize and then add hops and yeast.
Ya pretty much the only people I could see using the syrup as a mash are owners of an industrial syrup operation. Pretty sure you'd wanna use around a gallon of syrup per gallon of corn which would be like $200 worth of syrup to make probably less than a gallon of shine. I know that in industrial syrup production there's a byproduct they call "cooking syrup" which is basically the dark ends of the tanks that gets a little over heated, I could see someone using a bunch of that to make some farm liquor.
I just figured out the branding conundrum. The person who labeled said beverage was trying to be clever in a Madison Ave. kinda way. They conflated Grown Up with Apple and Grape to make Grapple. "The Grown Up Apple/Grape Drink!" ™
Hey folks, the donation widget is temporarily broken, so if you'd like to donate to help keep StP going with the paypal link below, I would sure appreciate it! There's a lot of new features I'm looking forward to bringing to the site (but unfortunately they cost money).