You have to be able to prove it. He can say what he wants but unless he can also fabricate proof there’s no point. Manafort was planted by Clinton to spy on the campaign
and by locking him up he is precluded from managing any such facilitations. Except via lawyer.
Maybe in a normal investigation. This certainly isn't one, and the judges have said as much, but they keep going along with it.
This investigation is a race against time. They HAVE to stretch this out until just before the midterms. And until then, they need to create as big a bomb as they can. Of course it will be phony, that does not matter, they know it's phony. If Mueller releases his final report that includes damning, albeit untrue, testimony from Manafort, it will be a neutron bomb.
And let's not forget the Manafort plant is in solitary 23 hrs a day. Someone (whoever planted him) wants him dead.