The Attraction processors were using the EAD "brains" for the surround processing. Not sure if they still are since EAD is out of business. My understanding is the two channel setup in these units is Sim Audio's design.
You are right! There are very few surround processors that I know of that come with balanced inputs. You might be able to get it as an option. Probably because Sim Audio has a bypass circuit in their two channel preamps thus the reason to have no balanced on the processors.