Why do people keep saying this? Sonnen was offered a fight where he had literally nothing to lose and everything to gain. Best case scenario for him would be he beats Jones and becomes LHW champion, makes a shit ton of money, and has his popularity/legacy/standing in MMA absolutely skyrocket. Worst case scenario is he collects a quick and hefty paycheck and loses a fight that nobody expects him to win anyway that has no impact whatsoever on his standing or how people perceive him as a fighter.
No balls necessary, anybody with a brain would have done the same in Sonnen's position.
Yeah stepping in to a fight with Jon Jones on 8 days notice would take no guts. He's moving up a weight class on 8 days notice, and while he would have a chance to win a title he also has a great chance to take a monumental ass kicking. Give your head a shake.