The issue is still not really speech.

Durov wasn't arrested because of things he said or things that were said on his platform, he was arrested because he refused to cooperate in criminal investigations while he allegedly knew they were happening on a platform he manages.

If you own a bar, you know people are dealing drugs in the backroom and you refuse to assist the police, you are guilty of aiding and abetting. Well, it's the same for Durov except he apparently also helped them process the money.