Bryan makes the same mistake as many other reviewers by measuring the focal length at close range.
At close range zooms "loose" focal length more rapidly than primes.
If he had only focussed say on something at a couple of hundred metres I am sure that the 570 he calculated would be much closer to the 600 of the prime he was checking against.
I'm not sure it was a 'mistake'. One would hope that testing at or close to infinity focus would yield a result of 600mm, within a reasonable rounding error - that's how the focal length sepcification is defined. A 600mm lens at 200 m frames an area of 12 x 8 m - most of us shooting with 600mm lenses don't shoot houses from 1/8-mile away. What a typical user would care about is the effective focal length at a typical subject distance (and a 600mm lens at 10 m is a 'typical' scenario for birds, for example).
If for AFMA it is advised to set the target at 25-50x the FL, I would use the same distance for this sort of measurements. That makes 15-30m for a 600mm lens.
I like Bryan's reviews but he is isolated in his suggestion to shoot at 500mm and upres. I find the lens plenty sharp at 600mm f/8 and never found the need or the convenience to do as he suggests. Other users/reviewers seem to be of the same opinion too.
I also have to say that I envy you for shooting birds at a typical distance of 10m. I rarely have the chance to get that close except for ducks, geese and other very tame birds in city parks and the likes. In fact I would say that 20-25m is more like my typical working distance.