He likes to photograph big birds which you can get close to. He also probably had access to 800mm lenses.
Unfortunately, shooting with a D1 X and a 100-400mmL plus 1.4X TC or a f/4 lens plus 2X TC won't autofocus, so you have to use the big dollar and ultra heavy lenses in the $11,000 range.
With a 1D MK IV and a 100-400mmL with 1.4X TC, you get autofocus and a 728mm equiv field of view with a low cost lens (relatively).
With a 1D X, you can get 400mm max and retain autofocus, so it really doesn't matter how good the AF is, if it doesn't work with your 100-400mmL and a TC!
Canon is trying to put their spin on the issue, but I doubt that many birders are happy.