Yep, he always does his GUMPs check but still it didn't save him from having the accident. There were many many factors involved so this "most preventable" of accidents becomes, on detailed analysis, much more complex. Can happen even to the some careful pilots and to those who "always do GUMPS". The best defence is to acknowledge that it certainly can happen to you, even if you "always" do the checks!
He mentions "sterile cockpit" which is another important tool to reduce human factor error at critical phases of flight.