TL;DR Because of Title 10 of US Legal Code
While it may be true that governors might themselves be sending guard members to help out fellow governors, the NATIONAL guard can be mobilized by either the state or federal government.
The official National Guard Fact Sheet gives several examples of situations where Federal deployment can take place, including this one, which may be applicable:
Air and Army National Guard. Air and Army National Guard can
specifically be called into Federal service in case of invasion,
rebellion, or inability to execute Federal law with active forces.
Three Methods of Deployment
From a writeup in the Atlantic:
There are essentially three ways a guardsman can be brought into an
active status to perform a mission. One method is called “state active
duty,” in which the governor activates state Guard members in support
of a particular mission. The state must bear the cost, and the members
are under the governor’s command.
Another method is when the federal government activates the Guard in
what is called Title 10 status—that is what is meant by “federalizing”
the Guard. The federal government pays, and activated Guard units are
placed under the control of the secretary of defense and the
president, with an active-duty military officer in the chain of
...the National Guard can be activated in a third way. Under Title 32
status, a guardsman is activated by and remains under the control of
the governor, but is paid for by the federal government. This helps
relieve the financial burden on the affected state.
From the article the OP linked to, it appears that the North Dakota case is the third way:
Mike Nowatzki, a spokesman for Gov. Doug Burgum, said the Department
of the Army made the request through the National Guard Bureau.
“We have monitored the ongoing crisis at the southern border and have
responded to the request by sending North Dakota National Guard
Soldiers to support the efforts to secure our border,” the Republican
governor said in a statement.
Nowatzki said the deployment is being funded by the federal
Title 10 Status
The "Title 10 status" referred to above comes from Title 10 § 246 of Federal Law:
(a) The militia of the United States consists of all able-bodied males
at least 17 years of age and, except as provided in section 313 of
title 32, under 45 years of age who are, or who have made a
declaration of intention to become, citizens of the United States and
of female citizens of the United States who are members of the
(b) The classes of the militia are—
(1) the organized
militia, which consists of the National Guard and the Naval Militia;
(2) the unorganized militia, which consists of the members of the
militia who are not members of the National Guard or the Naval
Disagreements Between DoD and State Governors
The ability of the Federal government to deploy the National Guard in a way contrary to a governor's wishes has sometimes led to litigation.
See for example this 1989 writeup in Air Force Magazine about some National Guard training deployments ordered by the DoD that state governors tried to prevent.