I'm not a legal expert, but this definition sounds like it
(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;
Now there are varying opinions on whether this detention is legal or not, but personally the Russian government is one of the last organizations I would trust in determining that.
Sure, we are talking about piracy in general, not Russian opinion.