I have a few thoughts here...
First, I don’t think Chris Kyle would have to make things up in order to sell books, so I’m inclined to believe the story. I read the book twice, I know there can be a bit of embellishment at times, but still...
Secondly, I’ve been on jury duty for a defamation case. There are two issues. You have to prove that 1) someone said something - whether true or false and 2) that you were damaged as a result.
If I say “You’re fat and ugly” - that may not be nice, but unless you’re damaged financially, it’s not defamation. So how is “The Body” damaged by any of this? Sure, when I read it, if Jesse said what Kyle says, I thought “man, what a douche” but then what?
Nothing.
And #3, it must be untrue.