I would disagree with that Zee, but I guess it would depend on how you're defining "majority" in this sense.
What I meant by "majority" is that the break-in debris created in the first 1,000 miles should be the majority of the total break-in wear over the first 5,000 or even 10,000 miles.
I agree that most engines aren't "fully broken in" until around 5K - 10K miles. I tracked the fuel mileage in my Tacoma from the day I bought it new, and around 5K-6K miles the gas mileage all of a sudden increased noticeably, which I chalked up to the engine concluding a full break-in since all other factors were pretty much constant (same oil brand & grade, same fuel, same daily commute routine to work, etc).
We see plenty of UOAs that show it can take multiple OCIs to get break-in wear to drop down to "normal" levels. That would indicate that break-in does not finish it's "majority" of wear in the first 1k miles. But what we don't know (and I comment in my post below), is how much is residual break-in metals which need flushed out, versus how much is "new" break-in still occurring?
Typical Blackstone type UOAs only see wear particles that are ~5u and smaller. On all but one of my new vehicles, which I typically did around 1,000 to 1,200 miles on the first OCI, I always see lots of "sparkles" in the oil on the first oil change. What surprised me was on my 5.0L Coyote V8 I did the first oil change at 2,200 miles, and the oil barely had any "sparkles" at all in the oil compared to other new vehicles I've done the first oil change on around 1,000-1,200 miles. Of course, the level and quality of machining and fitment of the parts on each engine can vary, but this Coyote runs very quiet (no typewriter tick, etc) and runs super strong. Could be it was a manufacturing QA unicorn.
Hard to say what the size of those"sparkle" particles are in the oil, but it's all the break-in debris that the oil filter can't catch and retain, and it's visible to the naked eyes so the majority of it must be larger than 5u. I'd bet most factory installed oil filters are not really very efficient, so a lot of the visible break-in "sparkle" in the oil should be particles much larger than 5u if they are visible to the naked eyes. The point is, on the next oil change there isn't any sign of "sparkles" in the oil, so that's why I say the majority of the break-in wear probably occurs in that first 1,000 miles. The rate of break-in in the first 1K miles must be higher than the break-in rate from miles 1,000 to 2,000 for instance. If it was constant, the OCI between 1K and 2K miles would look just a "sparkly" ... but I have never seen that ever happen. Even if I do the first OCI at 1K miles, and the second OCI at 5K miles, there is hardly any visible "sparkles" in the oil, which means the majority of that visible break-in wear was seen in the first 1K miles of break-in compared to the rate seen between 1K and 5K miles.
Obviously more break-in is happening after that first 1K miles and is still going on in the 5u or less size range, and still shows up on the UOA, but I'd bet the rate of the 5u and less break-in debris also falls off between a 1K and 2K OCI and beyond. I'd have to see a graph of UOA wear metals (in ppm/1000 miles format) to see if the wear metals break-in rate falls off after an initial 1K OCI. Do you have any data that shows the ppm/1000 miles on a vehicle that had an OCI and UOA at the 1K, 3K, 5K and 10K mile marks for example? That would show how the 5u and smaller wear rates change and fall-off with increased break-in mileage based on UOAs.