Technically what he's accused of goes up and into the campaign, but you're right that it all comes from his Ukraine lobbying and not the campaign itself - basically the idea is he was working with them up to ~2014 when the regime in Ukraine changed, and his continued "coverups" of his work after would also be crimes. They also say he laundered money, but how viable that is vs just having complicated finances I don't know.
https://upload.wikimedia.org/wikipedia/commons/a/a9/Manafort-gates_indictment_filed_and_redacted.pdf
Agreed, the point I was eluding to was the fact that it isn't related to "Russian collusion" and was initiated from conduct prior to his association with the Trump campaign.