Indeed not true. I should know, since I was one of the people that got hold of the Van Buren docs and released it to the public. You would see remnants of the Legion, but they were supposed to show up in force only in Fallout 4. The group was originally created by Josh sawyer when he was still at BIS because of his love on everything Roman.
And I was indeed attacked by water monsters in Lake Mead, but I had the Wild wasteland perk on.