From the Wiki Page:

"The Act does not prevent the Army National Guard or the Air National Guard under state authority from acting in a law enforcement capacity within its home state or in an adjacent state if invited by that state's governor. The United States Coast Guard (under the Department of Homeland Security) is not covered by the Act either, primarily because although it is an armed service, it also has a maritime law enforcement mission."

It's confusing because DC does not have a governor so it looks like an edge case that has not been tested before.