Absolutely not true! That is why we have shoulder harnesses - to prevent the upper body from moving forward. Could you have a neck injury from the noggin moving forward while the shoulders are restrained? Yes. But will your HEAD move violently forward and impact the seats, etc? No.
Also, I believe they were in an SUV. The most likely scenario is that the SUV attacked them out of malice, but that is just a guess...
Think thru the physical issues. Don’t be sarcastic unless you know your stuff-and you don’t.
Upper body does not equal head - and their injuries were brain stem and cranial. Shoulder harnesses do nothing for the head, especially if the car impacts while turning (ie skidding on ice). Bodies slew sideways upon impact and can and often do hit something to the side. You know those side curtain airbags Volvo promotes ? Why does your genius think they are pushed as a safety feature ?
Secondly, shoulder harnesses usually only “catch” after a few inches have paid out, especialy if the foot is not on the brake as is often the case in a skidding scenario. If a person is already leaning forward or otherwise causing slack in the belt, the shoulder harness will not prevent your upper body from moving forward enough to impact something. The girl was 7 at the time - if she wasn’t in a booster seat the likelihood is that the shoulder harness was behind her back. In a normal car configuration, the shoulder harness is configured for an adult and crosses a child’s neck or face. Most people do not use a shoulder harness with children at this age for that reason - they get a LATCH system if they know anything.
Additionally, another possible factor is seat collapse. Since seats are not safety-rated nearly the same as seat belts, seats are often a significant cause of injury if they move or collapse upon impact. If any of the seats in the car move, your seat belt cannot protect you.