Well, a child who is listing should be harnessed.
The argument is that there is more movement, but the body and head move together, so there's less head only movement. Plus, I bet that's the 10 year old dummy. A 10 year old has a well ossified skeleton, just a few years away from physical maturity to boot. That amount of movement is why three year olds should not be in boosters, since their bones aren't ossified. Not to mention, they're less likely to be in position.
I don't say that one is safer than the other. I say there are no data to show that a harness is safer than a booster, or the other way around.
Wendy