Being in Australia I haven't read any of his articles but if he is claiming one group of that size and not that it shoots groups that size consistently then I would say its entirely believable.
First group I ever shot out of my sako quad was CCI and it went .25 at 50y. At 100y long term it averages about 1.25 but it does throw up the odd .5 or so group.
I also have a marlin 1894 fitted with a 4x scope, loaded some up with 140gr FTX and first group went .5 for 5 shots at 100, fired another 3 groups with the same load and the averaged ended up 2" with only one of the other 3 groups going under 2".
Big difference between a rifle that will shoot .5 consistently and a rifle shooting one brilliant group by chance.