“...actually doing submarine espionage.” You have fallen for a disinformation propaganda piece if you believe that.
They were in at the beginning on Soviet KGB and GRU spying on the Manhattan Project. Multiple books, including by Soviet defectors and the Venona Project have verified their treason. See these articles:
https://www.historynet.com/the-venona-project.htm
https://www.osti.gov/opennet/manhattan-project-history/Events/1945-present/venona.htm
Or read this excellent account:
The Venona Secrets: Exposing Soviet Espionage and America’s Traitors (Cold War Classics), by Herbert Romerstein and Eric Breindel | Oct 1, 2001
I use to read the auto biographies of defecting Soviet Spies. That is where I heard about the spy switch. The Rosenberg’s were still guilty of treason, just not Atomic treason. The real spy went on to spy in England before retiring in Russia.