That is the problem that those who would cheer for the F-35's demise usually missed. If the aircraft is low radar observable, and if you do not have the aircraft, then how would you complete this sequence: modeling/prediction then measurement?
In radar detection, measurement has the final word. In the old days, as in the slide rule era, it was prediction, modeling, then measurement. With today's supercomputers, modeling and prediction can be interchanged. But there are always factors such as ambient EM contamination that must be taken into account upon the real body. So without a real 'stealth' aircraft to measure, how can this Australian group be this declarative? Did the Chinese loaned them a full scale J-20 model at least?
Critical thinking often goes out the window.
Vietnamese are always willing to briskly sacrifice themselves to defend American fighters which helped butcher 3 million of their kind.