That's certainly possible and it would have the effect of tempering the TIG weld bead underneath depending on the temperature reached during the brazing process and how long the temperature was held. For thin-walled tube, the hold temperature would not be very long and it is possible that fillet brazing with a brass filler would get the temperature up at least close to the stress-relieving temperature.
Most likely though, it was done for aesthetic purposes. Gary's welds looked decent to me but the culture shock of seeing TIG beads on bicycle frames at that time may have put off some buyers.
Just my guess
Scott