If someone needs to whistleblow about a classified thing they would have to do a closed session, yes. However, whistleblowing must reveal a crime or it doesn’t get protection. You can’t just go into congress, even closed session, and say; hey, did you know they have secret drones at this base! Likewise, even revealing aliens isn’t whistleblowing. You’d get prosecuted. HOWEVER, I have long maintained the reveal would be worth the chance of prosecution and, besides, you wouldn’t do that to congress behind closed doors, you’d go to the media (preferable after you were in another country).

This amendment does not mean someone can reveal alien ships IF they exist and are (presumably) classified.

It simply codifies that anyone coming forward to reveal criminal misdeeds won’t be retaliated against. It’s actually kinda boilerplate.