I guess they want to say that mechanism of interaction between the analyte and stationary phase is by partitioning... it is assumed to be partitioning when the plot of Log k' versus fraction of water (or % organic solvent) is linear
it is adsorption mechanism when Log k' versus Log fraction of water is linear...
by the way, did you find out about the pH of ammonium acetate in 50% MeOH? I'm very interested how it changes... did you measure? how did you do the calibration of pH meter?