It wasn't that there was any specific language in the rules, it was because we were able to use city codes when booking with paper tickets back then. With the change to etickets it became difficult, if not impossible, to use city codes and about the same time we started getting the interpretation that XXX-LHR, LGW-YYY, (along with other coterminals) was an extra segment.

Regardless, the same rule about using the point of origin has been in the rules unchanged all this time and we have speculated more than once about how to get around it including by using coterminals. Personally I believe, as with most of these rules we parse, that the airlines will interpret it in a way that benefits them and point of origin means city and travel through LHR and LGW (or LCY) will be travel through the point of origin. Whether or not travel through BWI will be allowed on a xonex originating in DC or EWR for an xonex originating New York, is more difficult to predict. I believe some airlines will interpret the rule in that manner. Personally I think with those airports physically located elsewhere like BWI or EWR, it is not travel through the point of origin as you are traveling through a different city (or state). One would want to ignore any talk of coterminals and argue that originating in a different city (or state) and then traveling through BWI or EWR is not travel through the point of origin.