No it's not. It is a federal reserve, still subject to federal laws, and some state laws.
What is the land reserved for? The Standing Rock Sioux Nation. It was reserved for their use as their nations tribal government was overthrown by the American government and their land was seized.
They were given the right to own their property in the treaty as individuals and as a collective. In the treaty it states this land is:
set apart for the absolute and undisturbed use and occupation of the Indians herein named, and for such other friendly tribes or individual Indians as from time to time they may be willing, with the consent of the United States, to admit amongst them; and
the United States now solemnly agrees that no persons except those herein designated and authorized so to do, and except such officers, agents, and employes of the Government as may be authorized to enter upon Indian reservations in discharge of duties enjoined by law, shall ever be permitted to pass over, settle upon, or reside in the territory described in this article, or in such territory as may be added to this reservation for the use of said Indians, and henceforth they will and do
hereby relinquish all claims or right in and to any portion of the United States or Territories, except such as is embraced within the limits aforesaid, and except as hereinafter provided.
Then the treaty goes through more stipulations and details about individual property rights, basically the process for deed registration, other items that were promised, etc, then it goes into the stipulations that the Sioux agree too and
here is where they are taking their liberties with the language:
the said Indians, further expressly agree:
They withdraw all pretence of opposition to the construction of the railroad now being built along the Platte River and westward to the Pacific Ocean, and they will not in future object to the construction of railroads, wagon-roads, mail-stations, or
other works of utility or necessity, which may be ordered or permitted by the laws of the United States. But should such roads or other works be constructed on the lands of their reservation, the Government will pay the tribe whatever amount of damage may be assessed by three disinterested commissioners to be appointed by the President for that purpose, one of said commissioners to be a chief or head-man of the tribe
Here's the thing though. If you start taking liberties with the word "utility" you could say it's "anything useful" by dictionary standards. No I think utility here should be used as is it "useful" to the people in that area. All the other examples, rail, roads, mail stations,are serving the local community where it is built. If we start applying it to "useful," in the broad sense, we may just as well throw out the treaty. A wind farm is "useful" so is a leather tannery, or a tire factory. But would you want it on your reservation next to where your kids play?
I think a work of utility is a flimsy leg to stand on when trying to get around this treaty and build an oil pipeline.