Their explanation is absurd, as pointed out.
I will disagree, if the ammunition in question has tar sealant around the bullet. This design, and all fluted chamber designs, require gas flow through the flutes for function. It is a mistake to believe that you want the case to stick to the chamber walls. It would be better if the case was frictionless. All the case should do is provide a gas seal; when cases stick in the chamber, it takes effort to pull them out. Sometimes too much effort. The flutes in the roller bolt designs float the upper 2/3 rds of the case off the chamber walls, the bottom third of the case provides the gas seal. If those flutes are clogged, you will have excessive breech friction and the gun will have failures to extract. Tar sealant will vaporize during powder combustion, and condense in the chamber, clogging the flutes.
If you search in the achieves, the first PTR rifles had deep HK fluting. But reloaders complained so much, they changed to a shallower fluting.
This is absurd. Do the German, Norwegian, Danish, Greek and Lithuanian armies only use Remington, Federal Eagle or Winchester ammunition in their rifles?
No one should ever think that first world Armies go to Tropical Jack's Surplus to buy ammo. They understand the operating characteristics of their weapons, at least better than an outsider, and they buy appropriate ammunition for their weapons. Most first world countries make their own ammunition for their own troops. I don’t know how the HK91 performed with LC ammo, but I suspect that LC was checked for interchangeability. I really doubt that German troops were ever issued anything but German ammo.
This is an interesting read. http://www.dtic.mil/ndia/2007smallarms/5_9_07/Geddes_1120am.pdf