Based on a bit of experience, and even more conjecture, I offer the following model, which I follow:
Some barrels need break in. Some do not. Your barrel will tell you what it needs, if you will listen. Shoot one shot and clean with a copper remover. Shoot a second shot and clean with a copper remover. If you're getting a lot of copper on the patch, you need to do a break-in. If not, go shooting and have fun.
When the bore is absolutely clean, only the first shot through the barrel counts as break-in. The microburrs and irregularities scrape copper off the jacket, and each successive shot builds up more copper. Only when the microburrs and irregularities are "naked" does the bullet passage smooth them down.
I have a 1917 milsurp Swede which copper fouled excessively. After just under 100 years of being shot, it still was not properly broken-in. I cleaned it very thoroughly, shot one shot, cleaned again, repeating for about a dozen shots. Then I shot three shots and cleaned, and repeated that a few times. I may have expended 30 cartridges. At the end of the treatment, I was getting very little copper on the patch. The rifle now will go a long time between cleanings.
Gale McMillan's article throws down the challenge for someone to explain what is gained by break-in. The answer is very simple: It allows you to go longer between cleanings. If a rifle is properly made in the first place, or broken in properly if it requires that, you can shoot more shots before copper fouling impairs accuracy.