Tough to tell because you didn't have Free Agency back then and it was more difficult to see how those players would perform on other teams. Almost all of them didn't do well elsewhere, although that could be credited to them being past their prime. He did have some key players that Jimmy took over like Irvin, Tuinea, Newton and Jeffcoat.
I think it was a combination of everything. And you have to give Landry credit, many coaches would not be open minded to using a computer based system to scout other teams and to scout players. Landry was all for it and the Cowboys were WELL ahead of the curve on using technology to improve the team.
Defensively, he reminds me a bit of Jim Boeheim. Both coaches came up with a defense they wanted to use and after awhile the defense was hardly new or innovative to their respective leagues. But, they knew how to run that defense better than anybody and knew exactly what type of players would fit the system and how to coach around that player's weakness.
Offensively, he popularized the use of the shotgun as a passing formation and had a ton of gadget plays.
I also think his ability to develop talent gets overlooked. No lack of discipline on Landry's teams. The O-Line is a good example. They had to stand up before every play, let all of the motioning go thru and were almost always a well disciplined unit. And it took foresight to turn Bob Hayes into a WR (he was a collegiate RB) and Rayfield Wright into an offensive tackle (he was a defensive player). Everson Walls didn't even start his senior year at Grambling. So, there had to be some plain ole coach teaching a player and figuring out how to make the most out of that player's ability.
YR