The term "all day long" is a hyperbole, and any normal person with a functioning brain will understand that... it is like saying the barrel was red hot (was it really glowing red? no), it is like saying a bullet hits like a ton of bricks (really? no not really).
I am not a good shooter. When I go to the range, I see some good shooters, and I hope someday to be one of them. But I routinely shoot 1" 100 yard groups with my plain-jane-totally-stock weatherby 243. I shoot off of a sandbag under the forestock, I don't use one under the butt. Magnification is 3-9X. This not a high end set up, it is not even a mid-range set up. When I say "routinely", I mean that out of 6 targets, 3 or 4 targest will be 1" or less. If the gun can do it on 4 out of 6 targets, it COULD have done it on 6 out of 6 targets, but the shooter was not up to the task.
The right way to define group size (but no one does it this way) is to use the methods used by artillery and air-launched weapons... things like 2 sigma probability of hit (also called the 95% radius), or Circular Error Probable.