Rum is made from sugar cane and spices, I will assume that rum sub. is alcohol(as all are) with the subbed spice flavors.?
The reason an extract would be called for is perhaps that 1T of extract=a LOT more of the real deal, and you have to adjust a recipe accordingly. The point is to get the flavor with out all the volume I guess.
With cooking, it is a little more forgiving, with baking, that is a lot more unforgiving. If it calls for extract, that is what I do, I just go with real, v. imitation at that point.
Just my $.02