No man's land is land which belongs to nobody. If you visit an international border, sometimes it will have two fences, one put there by each country, with no man's land in between. In the trench warfare of World War One, the land between the opposing trenches was no man's land, nobody ventured into it except at night, or when attacking. It was flattened by explosives, covered in barbed wire, the very image of desolation: this is the specific no man's land that the writer is referring to here. So, literally, no man's land is simply 'unoccupied territory'; in the context of trench warfare it is the most desolate part of the battlefield; metaphorically it is anything that looks like the latter, though in this context I think it is referring specifically to no man's land's role as the iconic image of trench warfare: flattened earth, barbed wire, corpses etc., the writer is praising photographers who went beyond what had become a visual cliché.