If you have a court cite that says as much, I'll be glad to read it.
Your own cite DOWNES v. BIDWELL, 182 U.S. 244 (1901) gives this:
The mere cession of the District of Columbia to the Federal government relinquished the authority of the states, but it did not take it out of the United States or from under the aegis of the Constitution. ... If, before the District was set off, Congress had passed an unconstitutional act affecting its inhabitants, it would have been void. If done after the District was created, it would have been equally void; in other words, Congress could not do indirectly, by carving out the District, what it could not do directly. The District still remained a part of the United States, protected by the Constitution. Indeed, it would have been a fanciful construction to hold that territory which had been once a part of the United States ceased to be such by being ceded directly to the Federal government.It may be added in this connection, that to put at rest all doubts regarding the applicability of the Constitution to the District of Columbia, Congress by the act of February 21, 1871 (16 Stat. at L. 419, 426, chap. 62, 34), specifically extended the Constitution and laws of the United States to this District.