Ghost isn't the only one who has seen those quotes. I've heard them too. He's not inventing anything. I just disagree with his sources (of those quotes)...
One common thread by most who have done the Grendel on long range elk is that the have a financial or family interest in a maker or sales of Grendel rifles. I know Alexander (of AAC) taped a hunt of him killing an elk at some godawful range. That was a stunt.
I don't disagree the Grendel is a very capable round, it's just not a moose or elk gun, and Ghost is agreeing there.
I would have no hesitation shooting a deer at 4-500 yards if the rifle was capable with a Grendel and good ammo, assuming I needed the meat. (But I really would do my damnedest to get to half that if I could)...
I also would have no concerns shooting an elk with a .264 win mag or a .26 nosler. Moose, well no. Not even with a fast 6.5, but I have better choices.