Yes, it's the brandy/rum/whiskey (depending on recipe) that gives fruit cake the flavor. Also, the big weight sources in a good fruit cake recipe are butter and candied fruits, neither of which will dehydrate. Dehydrating fruit cake will give you the equivalent of stale fruit cake, something that most people toss in the garbage.

Stale fruit cake can be rejuvenated by steaming a little and adding hard sauce. That basically turns the fruit cake into Christmas pudding. Hard sauce is made of butter and brown sugar creamed together, cream in booze of your choice (usually lots of brandy) and refrigerate until hard. Put a small scoop of hard sauce on the hot steamed fruit cake just before serving and watch it melt. It's certainly a concentrated calorie source!