He's holed up in Russia because the United States stranded him there.
He was in Hong Kong for the original reporting. When his identity was revealed the United States charged him under the Espionage Act, & revoked his passport. He was flying to Argentina through countries that didn't have extradition agreements with the United States. When he got off in Russia, his passport was confiscated as it was no-longer valid & he spent over a month in the Moscow airport, unable to leave or go anywhere else.
You're missing a rather interesting step in his adventures: staying at the RU Consulate for several days in Hong Kong. And then he ends up "stuck" in Moscow. Surely a coincidence.
He was in Hong Kong for the original reporting. When his identity was revealed the United States charged him under the Espionage Act, & revoked his passport. He was flying to Argentina through countries that didn't have extradition agreements with the United States. When he got off in Russia, his passport was confiscated as it was no-longer valid & he spent over a month in the Moscow airport, unable to leave or go anywhere else.