That could be true, I was advised by the Halfords team that 'generally' a baby should as you say be able to hold it's head but also, be of a substantial weight as well to make best of the car seat around 9kg was the advice I was given by the lady who fitted mine.
Link below to some advice I found, it does mention 9kg weight.
http://www.mumsnet.com/guides/car_se...ne_months.html
To be fair as well just because a baby can hold it's head up doesnt mean it's neck muscles are strong enough to field a front facing seat 'impact'. Rear wise they wouldnt really move except into the seat, forward facing IMO is best left till as late as possible. I believe my son was around 8-8.5kg when we turned him round merely because 1) he could hold his head and 2) was pushing against the rear seat with his legs and distressing him,