What is the calculated load and the overcurrent device rating? Sometimes aluminum is used to save money or it is specified by an engineer when the owner has requested that the engineer look for cost savings. You are right that a single set of copper cables will carry the output of the transformer and should be easier to install than double runs of aluminum.
To answer your question about individual conductors needing to meet 80% of the load, I do not believe that has ever been a requirement. You could certainly install parallel aluminum conductors smaller than the 4/0 as long as they combine to meet the requirement for the calculated load and the sizing of the overcurrent device to which they are connected.
