In some ofnthe reading i have done itnstatednthat aluminum had a specific heat of .90 and a heat capacity of 90. Granted i know very little about it, just seemed like the numbers supported using alum particularly for the weight savings.
Another reason for its use would be its heat sink capabilities in the summer, drawing heat out of the room.
I also thought that the aluminum would allow greater design flexibility (i.e. Raising the bench of the floor) and possibly reducing cubic footage needed compared to cob.
Currently we heat our 1900 sq ft house, solely with a relatively small soapstone box stove so i figured 5 ton of cob was a little overkill.