I'm not entirely sure what your asking either but I'll see if I can explain things a little.
The man in the video is Jeff Brozavich and he knows his stuff. What he is explaining in the video is how to setup your dies to make consistent ammo from round to round. You set your dies with cam over so they bottom out and square up. This helps to keep each round consistent in sizing and seating. Your neck is sized the same length down, the shoulder is bumped back the same amount and bullet seated to the same depth and all kept as straight as possible for each round. This is in relation to how you set your dies, so if the die is set to bump the shoulder .002, every round should come out to the same measurement or very close to. Same with seating the bullet, if you set the die to seat your bullet at an OAL measured to the ogive of 2.85 every round you make should come very close to 2.85 measured base to ogive. There is usually some variance and it's very hard to make every round dead nuts. My personal acceptable variance is +-.001 so I would be satisfied with rounds measuring from 2.84-2.86. Typically for me in my practices about 90% come out dead nuts and 10% have the +-.001. Now with this same ammo all measuring 2.85 from the base to the ogive you will most likely have a variance of a few thousandths from base to the tip of the bullet because the tips are inconsistent. In other words you can have 10 rounds that all measure exactly 2.85 base to ogive but there may be +-.003 or even more variance when measuring these same 10 rounds from base to tip.
The relation this bullet has to the lands or lead of your rifle is up to you to measure and determine. So staying with my example of 2.85 let's say that is .020 off the lands in your rifle. So that means if you seated the bullet at 2.870 (again measured base to ogive) that bullet would be touching the lands of your rifle at the bullet ogive. Though it may not be touching the bullet ogive at exactly the same spot as your bullet comparator due to differences in diameter between the tool and your lead.
This is why you go through the process of load development and seating depth test. You measure to find the round is touching lands at that 2.87 measurement then test different seating depths to find which shoots best. Typically you test from 0 off (touching lands) to -.100 (2.77 base to ogive) or more. Once you find the sweet spot that is what you load all your ammo to, in my example I determined the sweet spot was .020 off so all my ammo will be loaded to 2.85.
Now as far as throat erosion, it does happen and it does change the relation of the bullet ogive to the lands of your rifle. Typically it takes a few hundred rounds to measure any difference but there is a ton of variables to that and it's just something to keep in mind. Sometimes the seating depth sweet spot remains the same regardless of how much erosion has happened. Other times you need to chase the lands (seat bullets out further) to keep accuracy. All of it is for you to keep an eye on and determine, so if after say 400 rounds you see accuracy degrade and you know it's not an issue with barrel cleaning or equipment measuring the throat erosion and re testing seating depth based off your new measurement is one variable to check.
Sorry for such a long post but hopefully it helps to clear some things up for you.
