Without testifying, the government would have to prove criminal intent regarding wrongdoing, e.g., he either did something illegal and knew about it, or consciously avoided doing something about an act which he knew was illegal (the doctrine of "conscious avoidance"). By testifying, the government can get Corzine convicted of a crime merely by parsing his testimony and finding someone else to give a contrary account of what happened, in order to prove Corzine lied to Congress.
Clearly, the second avenue is much easier to prove.
Regardless of the content of his statements or the genuineness in his heart, Jon Corzine made himself much more likely to be prosecuted for false statements (Title 18 U.S.C. Section 1001), if not for a substantive financial crime like commodities fraud.
Eric Dixon LLC