Well he does have a point. Congress can not be trusted to keep a secret.
If they think that they can score some political points they will have blabbed the secret to their trusted press allies before the hearing has concluded.
But he isn't "obliged" to tell all of Congress, just the "Gang of 8", majority and minority leader in both houses of Congress, and chair and ranking member of the two congressional (House and Senate) intelligence committees.
FBI Director James Comey meets with Congress 'Gang of Eight' - March 10, 2017
"Obliged" in scare quotes because there is a legitimate constitutional separation of powers question about Congress poking around in executive branch business.
This "Ganag of eight" is also supposed to be briefed on covert military actions and assorted other matters.
It's important to remember the purpose of the intelligence committees. In the 1970's, the Church Committee investigated grave abuses of power in the name of national security. One of its core findings was that the complete lack of congressional oversight created fertile ground for executive overreach. It recommended that special committees in Congress be created to keep the intelligence and national security community within the confines of the law.
Gang of Eight Members Want a Bigger Gang | ACLU | April 1, 2010
I'm of a mind that Congressional oversight is a nothing more than a propaganda tool. The oversight is toothless. The NSA, CIA and FBI are essentially rogue to the extent they can keep their activites hidden, which is pretty darn far!