was charged with justifying terrorism and refused to plead guilty
To me this sounds like he was charged with justifying terrorism, not for refusing to plead guilty. Reading further the charge is based on “conversations with other inmates during which Miftakhov allegedly expressed support for a 2018 attack on a regional office of Russia’s Federal Security Service”. In reasonable legal system it would stand on very shaky legs given the other inmates have every reason to say whatever FSB told them to say otherwise they will be tortured as well. But this is russia.
Which is to say he wasn’t really charged for refusing to plead guilty, but in practice the FSB can just keep making up false charges and torturing him until he pleads guilty, so there’s very little difference. It might as well be an offense as you implied.