Gnu, I've been able to experience a few new vehicles over the last ten years and all have had similar instructions regarding the running-in period. As I understand it, in the past engines had a few rough edges from the casting that the running in removed, and these were flushed away with the first oil change. Now days the machining is so precise that issue doesn't happen.
I'm no engineer but what I was told in relation to aircraft engines was that full throttle builds pressure behind the piston rings which beds them in better against the cylinder walls. One of the light aircraft engine manufacturers published an article titled "don't baby your engine".