There may be nothing more to this than partisan politics, but anytime the director of the CIA says that the CIA has lied to Congress, there should be a full and thorough investigation. It will either expose Panetta as the liar or show that the CIA has in fact lied to Congress, which would be serious.
Congress can’t keep a secret. They leak like a sieve.
You can have a CIA that shares every detail with congress or you can have a CIA that is actively rooting out people that want to harm the USA. You just can’t have both.