Why confess?
There was no reason for Whitaker to respond to the question about his "opinion" on whether the flight attendant drank the booze, with anything other than "I have no opinion on that. I have no reason to have an opinion on that." The "evidence" that was provided, doesn't give any good reason to have one.
And even if one or ALL of the flight attendants had been falling-down drunk, what possible meaning could that have for the crash? None. The crash was caused by a mechanical failure due to bad maintenance.
But that wouldn't have been a "satisfying" end to the movie, I guess. And if Whitaker hadn't been drunk too at the hearing, perhaps he would have been able to not confess based on nothing.