This indicated to me that even these tapered factory rounds were not putting much pressure on the bolt face.
If the friction between case and chamber is high enough and internal pressures are low enough so the side walls don't stretch, then what you say will happen and I don't believe it depends on taper at all.
It is not like Ackley put his opinions and handbook out in the vacuum of outer space. He had a lot more eyeballs looking, reading his publications than ever will see this thread. He got replies, smart people told him what was going on.
He never tested his theories with high pressure cartridges, or at least never published it if he did.
When someone only publishes data that proves their theory, out of a universe of tests that would prove other wise, either he was really lucky, or real selective.