They use positive offset because it doesn't effect the frequency of the EMEM plasma devices. Loyd uses an Atelier Robin F165 to control the EMEM plasma. By default an F165 puts out a positive offset.
A positve offset is only more effective when used as a pad device. We decided this years ago because of Hulda Clark 's research.
More importantly (when using a plasma EMEM device) is to use a 75% or more positive duty cycle because it has more power and will not cut out at the higher frequencies. This can all be done with the F165.