Although they are Russo's words, I don't agree to their reasoning. I do, however, agree with their decision to pass on the symbol of Captain America to Sam.
Bucky is fighting for the right side now but that doesn't absolve him of the crimes he committed. Him being brainwashed doesn't erase his sins.
At the end of Civil War Bucky is still a criminal in the world. He has taken refuge in Wakanda and is healing. He doesn't leave Wakanda and fights with the Avengers and gets snapped. When he is blipped after 5 years and fights and wins with the Avengers saving humanity, however great his contribution may be, he still committed murder in his past and will be questioned as a criminal if not by law then by the media.
Bucky won't be an unquestioned symbol of integrity. His past and PTSD won't allow him to make the right decision at all times that is expected of Captain America.
Sam, on the other hand, fits the role perfectly. A retired war hero, who helps others with their PTSD, who got back into the field to help Captain America, when no one else would, was branded a fugitive and traitor, but did the right thing everytime no matter the personal cost.
He's the perfect candidate in terms of Steve because he sees himself in Sam.
He's the perfect candidate for the world, with unquestioned integrity.
Also, Steve had seen Bucky's mental and physical suffering and would rather let him rest and live out his days as a normal being. To get a life.