I haven't read the article at issue, but success as a sniper depends on three factors, the accuracy and reliability of the weapon and it's optics, the quality of ammunition, and the shooting skills and mental state of the shooter.
On the No4Mk1 T, the Cdn Army EMEI stipulates an accuracy standard of a 5 shots in a 3"x 3" group.

I've never fired one of these, but I'm sure that one properly set up will do better than this. I've certainly gotten better than this with handloads in both scoped No4 sporters and a pseudo-sniper LB No 4 7.62 target rifle with a 4x scope in an S&K mount.
I own and shoot 3 bona-fide sniper rifles, an 03A4 Springfield, an M1C Garand and an M1D Garand. They will all shoot between 1-2 MOA with quality handloads. Military ammo is a limitation on accuracy as it is tough to find 2 MOA or better accuracy with any standard military ammo. Optics are the other limitation. The WW2 era sniper rifles generally used low-power optics in the 2.5-3x range with thick recticles which makes it difficult to resolve the point of aim at extended, and often undetermined, ranges under varying light conditions. The original Weaver 330 scope on the 03A4 was an off-the -shelf solution which is a flimsy POS more suitable for a .22 rabbit gun. The Lyman Alaskan was a much better scope and a militarized version of it was used on both of the Garand sniper variants, and to some extent on both the 03A4 and the LB No4 spipers. It was still limited by it's low power.I'd love to have a scope on my M27 Finnish MN which will shoot close to MOA with quality handloads.
The training, ability, and mental state of the shooter himself is the other key determinant. A great target shot will not necessarily translate into a great sniper in field conditions where initiative, resourcefulness, endurance, concentration, and most-importantly, the ability to detach oneself emotionally from the repeated prospect of killing a specific human target, is important. I'm sure that most men would have a great deal of trouble with this. I once knew a gentleman who had been a sniper in WW1. He was a quiet, considerate and reserved man who divulged very little detail on his experiences other than to comment on the accuracy of the Ross sniper rifle and the physical discomfort and deprivations that were experienced in the trenches.
Re the comment on .303 ball ammo produced by WRA for the Brits, it was loaded with a ball propellant, which was later morphed into the BLC2 which we still have today. Early benchrest shooters used to like surplus lots of this propellant in the .222 Rem.