I have the Go video aswell so I know what your talking about. Although im not a 737 pilot yet (start type rating Jan) I think I can answer your question about autopliots on autolands. On a 2 autopilot plane like the 737 both autopliots are engaged for an autoland. This is so if one fails during the approach there is still something flying the plane. However if an autopilot fails during the approach above DA a missed approach would be executed as you must have at least two functioning autopilots to execute an autoland. But on plane with 3 autopilots 747 400 for example its the at the commanders discretions to continue the apprch if one of the autopilots pops out above DA, as they still have two functioning autopilots. These two different systems are known as fail-passive and fail-operational respectively.
As for why the Captian disengages the autopilot and Autothrottle at 6 miles in the video, this is because he is visual with the rwy and able to carry out an manual landing, which contrary to popular belief we will do whenever possible. Why let the autopilot have all the fun!!
Hope this helps