The theory I was taught was that under the report, the momentum of rearward movement, and the inertia of the bullet pressed into the cartridge (the casing being held in place and “tied to” the moving firearm) could cause the case to move backwards faster than the bullet, causing it to pull out slightly under recoil.
Its not a theory, its an observed fact. BUT bullet "pull" is rarely an issue for rifle shooters, it is something revolver shooters need to be concerned with.
With rifles and semi auto pistols the concern is bullets being pushed back into the case, not pulled out from recoil.
This is due to the way the different designs handle the ammunition. Ammo is only "tied to" the moving firearm in revolvers. Because the cylinder chambers also function as the magazine, and rounds in the cylinder are held in place during recoil by the case rim (or headspace ledge if rimless). When the revolver recoils, the case is held firm in the cylinder, and the case is pulled back during recoil, while the bullet is not, and it is inertia that holds the bullet in place while the gun in recoil pulles the case backwards, off the bullet.
This is called bullet creep or bullet jump, because it
appears the bullet had jumped forward, but the reality is the case was pulled rearward. The amount of "pull" depends on the balance between bullet mass (intertia) and the recoil force involved. Proper case neck tension is enough to hold the bullet in place when these forces are low. When they are higher, a crimp is needed. At very high levels a heavy roll crimp is needed.
Rifles do it differently. Pull is not the issue, being pushed in (and/or mashed) is the issue. Unlike a revolver cylinder, ammo in a magazine is not "tied to the gun". It "floats" in the magazine. There is some clearance between the round and the front and rear magazine walls.
During recoil, the entire round in a box magazine just "sits there", due to inertia. The rifle moves back under recoil but the ammo does not, UNTIL the rifle contacts it. This happens when the front wall of the magazine, moving rearward during recoil slams into the nose of the bullet, pushing the entire round back, and often (again, due to inertia) pushing the bullet deeper into the case. This also often results in deforming (mashing) soft nose bullets, IF the recoil forces involved are powerful enough to do it.
Additionally, there are two other situations specific to firearm type. One is the semi auto where it isn't just recoil but is also the bullet being slammed into the feed ramp during the loading cycle that tries to shove the bullet deeper into the case. The other is tube magazine rifles. Here you have the combination of the entire column of ammo, lined up bullet nose against the case ahead of it "slopping back and forth" under recoil forces, PLUS spring tension slamming the rounds to a sudden stop during the feeding cycle.
These are the reasons we crimp ammunition (roll or taper and amount used) when NEEDED due to the recoil and operating characteristics of the firearm it is being used in.
When these don't apply, we don't need to crimp. Proper neck tension alone is enough to do the job. A light recoiling repeater, such as the 6.5mm bolt action mentioned, ordinarily doesn't need crimped bullets.
The other thing to remember about crimp, when its done right (and needed) it is a benefit. But when its not done right, it can be worse than no crimp and done really badly (such as in the wrong place) is a disaster that can make ammo unchamberable and unusable.
Hope this helps you understand why some rounds get crimped and others don't.