President Trump and his new transportation secretary, Sean P. Duffy, made it sound as if their power to pull the plug on New York City’s six-week-old congestion pricing program was absolute.

Mr. Duffy, who has been on the job for less than a month, wrote to New York’s governor on Wednesday that “I have concluded” that the tolling program, implemented after a grueling, yearslong process, was not “eligible” under the federal statute used to enact it.

Mr. Trump’s explanation was even less complex: He hit the caps-lock button and invoked his authority as “king.”

“CONGESTION PRICING IS DEAD, Manhattan,” the president wrote on social media, “and all of New York is SAVED. LONG LIVE THE KING!”

The law, however, is far more nuanced. And legal experts say that in this case, it does not appear to be in the president’s favor.

“Declaring ‘I’m the king’ is not sufficient grounds for reversing” the program, said Robert L. Glicksman, a professor of environmental and administrative law at George Washington University Law School.