Apparently that's wrong. I have all possible sebillian operators from phase 2, but still miss one sebillian data hack from phase 1. And if I understood "researches.xml" correctly there are no triggers for datahacks beside operators from same phase. For example, there is only single instance of "UnlockKnowledge( "Researches.SebillianDataHack1" in whole file and it triggers only from phase 1 operators and chieftain:
EDIT:
After another inspection I'm right about only Light Scout Operators from phase 1, which don't have older brothers from higher phases. Scout = Landing ship = Carrirer = Battleship, Corvette = Cruiser = Dreadnaught, SmallBase = MediumBase = LargeBase, etc.