Or that 5.56 is made to injure so it takes 4 soldiers out of the battle to carry the wounded one.
The amount of ignorance being spread and accepted can be pretty bad sometime.
It probably became an urban legend back in the 80's when people wondered why they moved from 762 to 556, so they came up with this reasoning.
But with the Russian and especially the Chinese are mass producing ceramic plates, this urban legend may become partially true in future conflict with near peers.
The reason the US is specifying 6.8, a 7.62 sized cartridge, in the NGSW, is to have a cartridge that can load a bullet with enough velocity and section density to defeat ceramic plates.
But with the volume of 6.8 cartridges, one simply cannot carry as much ammo as 5.56. The 6.8 has almost the same volume as 7.62. The availability of armour piecing weapon will also de-incentivize opponents to wear their heavy armour plates. The opponents will start trading off heavy armour for speed of manoeuvre, knowing that one side has less ammo available to restrict movement and last the fight.
The US is already saying the NGSW will change the way infantry fights. My opinion is that 5.56 will stay around to complement the 6.8 SAW. 5.56 will keep the enemies wear their armour and restrict their speed and movements, so the NGSW ( and a DMR in 6.8 )could do the killing. In a sense, 5.56 will be repurposed to "degrading" enemies than actually being expected to kill with shots in the COM.
This thinking is already sort of manifesting in the HK 417 as the urgent fielding of 7.62 squad DMR in the US army. This is because of the urgency of introducing AP capability through the 762 Tungten core Xm1185. The long term solution is the 6.8.