The Swift was developed in steel originally, and that is still ongoing. Xootr bought a license to mass-produce it and their material of choice was aluminium due to easier mass manufacturability. Peter verified the frame parameters in the aluminium to make sure it would still work. Some things were modified to suit aluminium, such as seat post diameter and fork legs diameter.
Italics are me filling in the blanks in my knowledge, ie speculation.
By 'fork legs' do you mean the front forks? Mine Xootr front forks are steel...