Why would Libby lie, as he seemingly did?
And is Fitz trying to sweat Libby to get more indictments?
The most plausible answer is the one Fitz - in an admirable exercise of restraint - refused to presume. He knew he shouldn't have given out the info (hence the 'non-secure line' comment), so he was trying to cover it up and didn't think he'd get caught. It is far more plausible than that he 'forgot' over the course of a few days, or that 7 or 8 others are all lying and Scooter is telling the truth.