Of course there's question as to whether the spirit of Section 215 is constitutional. It may be reasonable for the government to force you no to say something.
But can the government force you to knowingly make a false statement?
No, but they can punish you for telling the truth. The fact that you'll be punished for lying does not negate the punishment for telling the truth, just as the fact that you'll be punished for telling the truth does not give you an excuse to lie.
There are no checks against their power-- no court, no congressional group, and no group of people is willing to regulate the "watchmen".
This is the default state of affairs, and it only changes when there is strong public outrage and pressure for the government to do their damn job of watching over the watchmen. Which is seriously screwed up.
I'm inclined to doubt that they can. But they can keep you locked up for a decade while the case makes its' way to the Supreme Court.