Why he testified at all or didn't invoke the 5th amendment is beyond me.
No doubt he was subpoenaed to appear and couldn't risk ducking the subpoena.
As for invoking his Fifth Amendment privilege: Suppose, after being asked if he planned the murders of the heads of the Five Families, he replied, "I decline to answer that question on the grounds that my answer might tend to incriminate me." Sure, he'd avoid perjuring himself. But, everyone would know that the reason his answer "might tend to incriminate me" was that he did plan the murders.
