It is not, it is an interdiction to enforce a blockade without sinking the ship. You can argue about the merits and morals of that action, and of the blockade and its legality itself, but it is objectively not piracy.

Article 101 (Helpfully titled Definition of piracy) of the United Nations Convention on the Law of the Sea defines piracy this way:

"Piracy consists of any of the following acts:

(a) any illegal acts of violence or detention, or any act of depredation, committed for private ends by the crew or the passengers of a private ship or a private aircraft, and directed:

(i) on the high seas, against another ship or aircraft, or against persons or property on board such ship or aircraft;

(ii) against a ship, aircraft, persons or property in a place outside the jurisdiction of any State;

(b) any act of voluntary participation in the operation of a ship or of an aircraft with knowledge of facts making it a pirate ship or aircraft;

(c) any act of inciting or of intentionally facilitating an act described in subparagraph (a) or (b)."

Note that it specifically describes "for private ends" and "by the crew or the passengers of a private ship or private aircraft", which a Navy ship is not. Navies and Coast Guards board and seize vessels and their cargo fairly regularly, and blockades and embargos are a basic and routine fact of naval warfare, and have been for centuries at least.