Resurrecting this as a result of researching 531 Millenium.
The reason regular 531 isn't welded is not because of any difference in the composition of the metal. It is because it is drawn to such thin sections that welding would introduce too much heat into the joints, distort the material and cause stress points and missalignment of the finished frame.
531 Millenium would appear to have shorter thicker butted sections to alleviate this issue without raising the weight of the tubeset significantly.