Bernie promises alot of things. Goes with being part of the Washington DC establishment for over 30 years
If anyone is going to release UFO documents, it would probably be the Trumpster. The DC Establishment, "The Swamp", "The Deep State", etc and The Donald have been at war with each other the last 3 years. If Trump loses in 2020, I could see him declassifying all sorts of stuff as his way of giving "The Finger" to "The Deep State" on his way out the door. If he's re-elected, I suspect Trump might slow walk some classified information into the public domain