Seamless tubing can be of any alloy used. It is drawn through a machine at high temperature, as opposed to being rolled over a form and contact welded together at high temp. Seamless tubing was just the new way of doing things and they had decals to tout that their bikes were being made this "new" way. Schwinn was not a pioneer in seamless tubing. There is a video I've seen in the VCC library that shows it being done. Ingots in one side and tubing out the other.
Kind of like the progression of tech in cars during the 80's when "fuel injection" "EFI" and those kinds of badges were on the backs of cars. Once it Became the norm, the badges went away.
Considering that the World and other lightweight models were being made with electric welded construction and the Continentals weren't, Id say its an easy step to say that brazing unlugged frames out of mild steel makes little sense. it would make them much heavier than a electro-welded frame. The brazing material is brass/bronze which is heavier stuff to begin with. There would be no advantage to this.
Now, how much of the frame was Chro-Mo is another story. The Super Sports in the 60's thru early 70s are "said" to have only the main tubing Chro-Mo and the stays mild steel. They are 30 pound bikes with much lighter components on them than these old Continentals, so I tend to believe that.
Anybody got a mass-spectrometer?