Green's depiction of Southern lower class Americana is unsentimental, austere and straightforward. He wants the audience to be immersed in the volatile world Gary and Joe inhabit. There are some understated flourishes and several instances of visual poetry but for the most part Green keeps things taut and unsentimental. Thankfully David Gordon Green understands this his approach to directing the film is subtle and organic, allowing the actors to shine first and foremost. This kind of unsentimental character piece needs a small tight focus so all of the nuances of said characters shines through. Joe is the kind of film that proves that a small story can be much more meaningful than a larger one. The conflict of the story however is not whether or not Joe lives but if he can save the future of a promising child, named Gary. Even though he thinks five steps ahead of the average man it is only delaying the inevitable. Joe is a man that knows he is stuck he has no where to go because his surroundings can't let him.
The line is in reference to the men Joe works with and in many ways applies to the titular character himself.
Forgive me if I'm paraphrasing but it goes something like 'These men have no more frontiers'. There's a quote about midway through David Gordon Green's Joe that I believe is crucial to understanding the film's thematic core.