I was told the same thing when I was in the Army.
Whoever wrote that was only partially right: the M60 and the FG42 had some features in common but the FG42 was made of high quality material, was select-fire weapon, and fed from a side-mounted magazine. Nonetheless, the FG42 was considered uncontrollable and operationally a failure.
The Johnson M1941 machine gun was recoil-operated, had a multi-lugged rotating bolt (which was copied for the AR-15/M-16) and also fed from a side magazine. It was also made of high quality steel forgings. No resemblance at all.
The M60 is made of cheap stampings, castings, and plastic. We used to have chipped bolt lugs, sear notch failures -runaway guns- broken firing pins, melted barrels, etc. Worse, it isn’t at all accurate with its open bolt/long slide until it fires, wobbly barrel attachment, loose fit to the pintle, etc.
I remember writing my congressman back then, while I was still in combat, asking why our weapons - the M16 and the M60 - were so substandard but I never got an answer.
The answer was that they didn’t care what we used. We weren’t their kids.