That really doesn't mean much. Hayne lacked the cardiovascular fitness required as he had just switched from NFL. He had the skills, but not the level of fitness required, and not enough time to get to the level required.
Semi Radradra didn't get much game time because the Fiji 7's squad has been together for years, and spent years together playing, training and preparing. Radradra flew directly from England to Tokyo to join the squad. If Radradra was committed to 7's and the 7's circuit, obviously he would be one of the first players picked.
Rugby Union 7's is not like league where they pick a rep team at the end of the year from the 13 a side version and then spend a week training before playing. They play and train together with the pretty much the same squad for years leading up to the Olympics.