*ALL CREDIT GOES TO 17GINGERNINJA*
I was recently watching a new video from NGT and this is what he said in the comments...
My theory is that Marlton was part of the original gang with Richtofen,Tank, Nikolai and Takeo, there is loads of evidence that leads to it aswell. For instance; he says "They asked me to leave? I was the one who reported the scuffing behavior...makes no sense". Just like when you play as the original characters (Richtofen etc..) they are always arguing and blaming eachother for them being surrounded by zombies and constantly being teleported.
Also in Kino, there are pictures of each of the characters, then there is one at the end that is a silhouette, and when you press X on it, someone says "Till we meet again friend" which leads me to believe that they did meet again on Moon, where he was asked to get away from them i.e. return to earth and maybe he arrived in Nuketown, which is why the moon easter egg is going on at the same time as Nuketown zombies.