"iron body" exist even in the real world,shaolin monks pratice that, and there's no haki or devil's fruit in the real world..they are just tecniques..haki himself is "just a tecnique",not an omni-inclusive power from which derive all the onepiece powers.
tekkai and shigan harden the body of the user,busoushoku creates a "spirit shield" which can be used for defense and offense..they are not the same tecnique,not even related.
Your telling a guy above not to follow the anime when you yourself are following fanboyish theories that have nothing to do with one piece.
Rokushiki of the six forms are techniques in one piece, Devil fruits and Haki are powers in one piece that can be utilized and or manipulated depending on the user with their own techniques.
Deal with it.