Copper fouling can be removed at anytime with JB Bore cleaner and improve the accuracy of a rifle. That's why I included the part for used barrels, and it's something I highly recommend to all of the Mosin, Mauser, and Endfield fans out there (or any well used rifle). The cleaning with the brush, patch, and solvent between rounds on a new barrel removes some fouling but also debris such as slivers and metal particles that result from the bullet contacting burrs and tool marks during the previous shot. By "debris" I mean very small particles but stuff that you still don't want in your barrel when a new bullet is propelled down the barrel. The JB cleaning removes almost all copper fouling and allows for direct contact between the next bullet and the barrel (especially the start of the lands) rather than the bullet contacting the copper fouling that is there from the previous shots. I have to believe that is a good thing.
You can wait 50 rounds or a 1000 rounds to clean the fouling? You could also wait 50 or a 1000 rounds before you tighten your scope mounts, but why would you? You could also wait 50 or a 1000 rounds before removing any debris in you barrel, but again why would you?
This might not be an issue on hand lapped barrels? Those barrels should be free of tool marks and burrs from the manufacturing process? But how many new guns come with hand lapped barrels?
Now as for why you can never know if it makes a difference. You can take a rifle right of the box, shoot it normally, and it will either shoot good groups or not. You can't know if that break in process would have made a difference because you didn't do it. Similarly, you could go through all of that and it will still either shoot good groups or not. You can't know if it improved your accuracy any because you can't know how it shot without the break in process. It's a catch 22. However, I do know that it made a world of difference on a very used gun.
I'm also guessing that the man this information came from knows a lot more about accuracy than I could ever hope to. The day I bought my first can of JB to try on that old Enfield I told a local gunsmith (who I bought the JB from) about that article and what I intended to do. He went on to say that Kenny Jarret was over rated and that the whole thing was a bunch of B.S. A couple of hours later I had a 1944 Lee Enfield with factory ammo out shooting every gun at the range. I know for certain that his advice made a difference on that gun, and I truly believe that it made a difference on all my new guns as well.