Yeah, I think he’s conflating the applications of the 7.62x51 and 7.62x54 (medium MGs and sniper rifles) with the 7.62x39 (asault rifles and SAWs). The Russians dropped the calibre of their service “AK” round to 5.45 in the late 70s to provide more ammo carrying capacity and maybe partly in a fit of copycatting (since NATO had gone to 5.56).
Regarding relative reliability/accuracy of one design or another, any indication if the 5.56NATO versions of the Finnish Valmet or Israeli Galil (Kalashnikov mechanism, Western manufacturing) are particularly notable for one or the other vs. the “native” AK or the M16?