What all this means is that Congress is not limited to the *other* enumerated powers delegated to them in Art. I Sec. 8, it does not mean they are not subject to the Bill of Rights, not even the 10th amendment, since they have been delegated exclusive legislative power over the district. The people of the district are still part of the people of the United States, whose RKBA shall not be infringed, who have the right to petiotion the government and to freely assemble, and so forth.
Art. I Sec 8 grants them the power:
"To exercise exclusive legislation in all cases whatsoever, over such District (not exceeding ten miles square) as may, by cession of particular states, and the acceptance of Congress, become the seat of the government of the United States, and to exercise like authority over all places purchased by the consent of the legislature of the state in which the same shall be, for the erection of forts, magazines, arsenals, dockyards, and other needful buildings;"
Art IV
states:
The Congress shall have power to dispose of and make all needful rules and regulations respecting the territory or other property belonging to the United States
It's not clear to me wether DC is a terrority belonging to the United States, but it hardly matters. This delegated power is what gives Congress the ability to legislate on general matters, as opposed to those enumerated in Art I, in respect to territories not part of any state.
Nonetheless, the Bill of Rights are amendments, and supercede the basic articles if there is a conflict.