That is a very general statement (broad brush?) and likely isn't based on published research or data.
If you are loading a progressive press and placing the bullets by hand, you can feel when a case is lacking neck tension as you are placing the bullet into the case mouth.
If you are using a bullet feeder, you'll feel the difference in effort required when seating the bullet. If you miss that, you'll likely see it as the bullet travels to the crimping station..
I have both set ups on my bench (Hornady LNL AP and Dillon 750) and have never missed a cartridge which was lacking neck tension
I know what your saying and I agree. I had some 9mm cases that no matter what I did to them I could not get any neck tension on them. They were all the same headstamp and all that headstamp got scrapped when I found those four or five cases.
I found them at the seating die.
I use a tension gauge to check my neck tension with.
I quit loading mixed 9mm brass quite a while ago due to problems like this and the fact that quality control was just to difficult with it.
Sorting brass for in a progressive helps me feel bad brass in my resizing die.
The feel of the resizing die is also more consistent.
The only way I feel it in my seating die is if I resize ahead of time. Then it's easier to feel something that is out of sorts.
Sometimes it's my imagination and sometimes it isn't. I also spot check with this tension gauge the same time I spot check my powder.
I learned a long time ago, when I feel something odd in my LNL-AP, whether I think it's my imagination or not, it get's checked.
I pick out the difficult to resize cases at the sizing die and scrap them also, I figure they have been shot to many times and they cause OAL issues.
I can usually find a split case in the resizing die also and if I miss it there I usually find it in seating die due to the way the bullet seats.
You would be amazed as to what you can feel in a progressive press once you tune in on it.