So if he had just taken the Fifth, there wouldn’t have been a case against him?
The Fifth is the way of the spineless infidel pig, whereas lying through your teeth in defense of Islam and Mighty Mo (taquiya) is a blessing.
Even if you're such a blithering dolt that your lies cause even hardened FBI agents to snicker.
Also I'd guess the federal judges at Alexandria Court have long been sensitized to taquiya by Moussouai, who told multiple whoppers so often that they thought he might be insane.
In the end they decided he was just another lying POS Muslim and threw him into Florence ADX (Supermax) with his other nutso cronies.