I believe you have the most correct understanding. Shoulder harnesses do a very poor job of restricting any upper body motion other then forward. A well designed racing seat restricts the other directions much better, Surprised a racing seat manufacturer doesn't plainly state that. I also believe one if the reasons the "Y" shoulder belt was no longer approved is, in a sideways incident, the Y shoulder harness induced a lot of forces on the neck in trying to restrain sideways body movement. the neck is not a good place to apply force to restrain body movement. Same applies to our typical shoulder harness today. The Lap belt is the key component here.

The BUtler quote also mentions the submarine belt purpose. Consider, if a typical belted driver weighs 200lbs, the upper torso/head has approx 40%? of the upper body mass restrained in a forward direction by the shoulder harnesses, or 80lbs. In a 10g forward impact, that stresses the two shoulder harnesses with 800lbs of force. Assuming no frictional loss going over the shoulder, that means ea end (4) of ea shoulder harness sees 200lbs of force during the impact. That means the lap belt has additional forces pulling up (parallel to to the spine?) of 400lbs. If the lap belt is mounted at its intended angle, the lap belt wants to rotate upwards with this lower shoulder belt induced force. The abdomen, above the pelvis, is very delicate with a lot of fragile organs, and a dangerous area to impact. That upward movement is mainly resisted by a properly positioned "submarine" belt. IMO a submarine belt is mis labeled for upright seating.