Might be he was working with FBI or he wasn't, but it doesnt change the fact that he was very much pro-Putin regime and had connections in local agencies. And he isnt a first one to get imprisoned just higher profile one. So even if you're literally part of their system someone can always come for you and eat you whole.
The only "The Untouchable Russian Hackers" who exist are ones who officially work for government, but they are not any different than NSA.
Charged with treason for aiding foreign intelligence agencies, odds are he was working with the FBI and is guilty as charged.
Group-IB was kind of famous for causing problems, it’s surprising it took as long as it did for the government to crack down.