The intent of Picasso's Guernica was to show the horrors of the Spanish civil war.
And as such, it (still) is both journalism and art.
Capa's intent was to show us the Spanish civil war, with all the horror and whatever else can be experienced in war.
Whether his pictures are art is another discussion.
But journalism they were, even if this one was staged. It conveys a sense of what war is like. And that's all it needs to do.
So what if it would be staged?
Verbal journalism (even accompanied by pictures) is not any more true. It always conveys a certain view on events.
A faked photo of an anonimous event not captured on film can be as valuable as any eye witness report. It can (and does) give us a sense of what, in the reporter's view on things, is happening.
Not good if you want to know whom exactly, at what place exactly, and at what time exactly, was shot, and how exactly. Sure.
But was that the reporter's goal?