I can agree with you here the FG42 is not truly an AR(from our modern concept) but if you put into that category it is a poor one but it was trying to perform the same role that an AR does (the light MG is ill advised but other AR also tried to fill this) so I see no foul in calling an AR.You will also see it found in most every technical source or book listed as an AR even Ian V. Hogg one of the leading firearms/military equipment writers of his time classifies the FG42 as an AR.I know that the weapon was supposed to fill multiple roles for paratroops but by and large any weapon that attempts to combine the roles of an SMG and a rifle can be considered an AR or you could call it an OCTFTMRFPKEW:Overly Complex Attempting To Fill Too Many Roles For Paratroopers Kinetic Energy Weapon if you want to.
