Interesting loophole. Would it only apply to DC residents, or to anybody in the DC? That could lead to some strange rules, like residents can not posess weapons but commuters from the more expensive (and whiter) Virginia and Maryland suburbs can!
Although I haven't considered all of the implications, I am pretty sure that it would apply to anyone actually
in the District. This is consistent with the restriction on transportation of firearms between the States and the requirement that "carry" rules be abided by even my non-residents.