To take your example "Trump declares that Rhode Island is not a state"—of course no rational person agrees that has any validity.
But Trump might then cut off federal funding to Rhode Island, with real-world consequences for their education, infrastructure, health etc. departments.
The statement itself is ludicrous, of course, but it establishes a parallel reality in which he can take actions with real-world consequences and federal appointees in the legal system might take his side.