But Buckbeak was never dead, remember when they went back in time, Harry and Hermione saw executioner cutting the pumpkin. Obviously, turning the time back is very complicated, but before they even went back in time, somebody (later we find out Harry and Hermione were those people) did all those things (warning them about Dumbledore and Fudge coming, Patronus to save Sirius and Harry).. so Buckbeak was never dead, Harry and friends only thought he was.