Why do you think that this isn't possible? You do realize that 1 MOA is a roughly 6.25" group at 600 yards, right? This is more than doable as long as the shooter is good at doping for wind, has a parallax free scope (adjustable objective/side focus), and mirage isn't too bad.

And what is there about his statement to agree or disagree with? If he's shot it at 600 yards, and been able to maintain sub-MOA groups at 600 yards, then he's stating a fact, not an opinion, which means there is nothing do disagree with... unless you're calling him a liar.

Mike