The Line Around A Future History

Trawling through Poul Anderson collections discloses some lesser known works connected to better known works:

a prequel to Tau Zero;
a sequel to The High Crusade;
a story that refers to the planet Nerthus and the Coordination Service of the Psychotechnic History.

Why is this third work, "Symmetry," not listed in the chronology of that History? 

At the end of the story, its protagonist, Dunham, or rather two duplicate Dunhams, are about to return to civilization with a matter duplication device that will revolutionize the economy and the subsequent course of history. No such device or its consequences are mentioned anywhere in the History. The story can be incorporated into the History only on the assumptions that:

(i) both Dunhams died accidentally before reaching civilization;
(ii) their spaceship containing the duplication device was never found;
(iii) no other explorer found the alien-made duplication technology;
(iv) no human engineers found how to duplicate matter although duplication was possible and was something that they hoped to find.

But the implication of the ending of the story is that the Dunhams will return to civilization.

The story's only point is a technical problem. The Dunhams find themselves in a symmetrical room and each have the same thoughts because their experiences have not yet diverged enough to make them different people. They are trapped in the room unless they open the door but the door knob is enclosed in a transparent cylinder that will allow only one hand to reach in and turn the knob. Every time one Dunham tries, the other tries as well so neither can open the door. How can they solve this problem? They do solve it and thus the story implies that they will successfully travel home. As background for this story, Anderson used references to features of the Stellar Union period of the Psychotechnic History but that does not mean that this story is set in that History.

As with Heinlein's Future History, on which Anderson modeled his Psychotechnic History, there is a line between stories that are in or not in the History. Stories can be very close to the line but nevertheless on the other side of it.