When those things first came out, I had a really bad feeling about it. Obviously, the designer was not an engineer or had little knowledge in metallurgy.
In smokeless muzzleloaders we use a plug rather than a cap. A cap stretches when pressure increases, causing a loss of thread engagement. SML's have a plug threaded into the barrel with the barrel threaded into the reciever which allows another ring of strength. The video speaks of pressure in the mid 80kpsi range.
I routinely shoot my 40 cal SML at 65kpsi plus with never an issue. How do I know this ? I have pressure trace equipment that I purchased for safety when shooting large loads that have never been fired in any rifle. In my pressure test gun, which has a large shank Savage thread and the entire barrel is 1.385" from reciever to brake. I built it heavy because I knew it would have its limits tested at some point. A seasoned veteran fellow ml'er at one of our shoots, loaded an unknown charge in my pressure test gun. The results were amazing. The pressure trace equipment peaked at 118kpsi. The breech plug did not move and the threads remained intact. However, it did bulge the barrel 5 thousandths of an inch right behind the loaded position if the bullet. I have no doubt, that had it used a cap rather than a plug, the results would have been much like the video.