It seems quite strange that when x470 is supposed to also launch that they'd be using the lowest end chipset with a severely outdated bios to test.
CPC Hardware already explained why are using A320 instead X370/X470: To avoid the omnipresent cheats in the X370/X470's BIOS ("Pour éviter les cheats à la con omniprésents dans les BIOS des X370/X470").
The overclockingmadeinfrance magazine also mentions that using a 400-series mobo wouldn't really change the performance or efficiency results, performance would change only 1 or 2% top maximum ("Cela ne change pas beaucoup les performances, 1 à 2% grand maximum").