Frankie already gave an affidavite about the family in which he already broke the silence. So I do not see the problem with him testifying. In this case he got to deny his previous statements. It seems that the scene in the government apartmemt was for dramatic effect.
Its one thing to give an affidavit to the Authorities, and it is quite another to go to a publichearing and announce what you know to the world.
"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"
