..and the reason the English fruit couldn't be the progenitor of the Chinese variety is?
Because it wouldn't be politically correct.
given that this is a DNA-related study, i would assume there are markers on some chromosomes which would establish which was the originating species/variety and any deviations. The article doesn't offer much on the DNA evidence itself, but this is one of the things that DNA studies are good for if your sample size is big enough. Notable that they authors of the study themselves had erroneous pre-conceptions going into this it appears/is alleged.