With someone as big and strong as James, it is partially about being big. James isn't stupid and neither are the Heat: If a player as physically over matched as Green is on him, he'll take him into the post. Green is a strong post defender for his size, but his size isn't enough against James and the only reason it didn't show up in the stat (according to you), is because it was limited sample size.
I didn't say Leonard didn't need help, I said he needs less help in the post. More importantly, James didn't play damn near exclusively in the post against him, like he did Green.