Concerning the fact that the historical records said that several members of the Justice League travelled to the future but only two went back, and that Supergirl was never seen again after her twenty-first birthday, Exact Words may apply if that was all that they said and Braniac 5 merely assumed it meant that Supergirl would die in the future. They might not have said that Supergirl was never seen again after her 21st birthday and that only two members of the Justice League returned to the present because she chose to stay in the future, since it reminded her of her home and she had fallen in love with Braniac 5.
Concerning the fact that the historical records said that several members of the Justice League travelled to the future but only two went back, and that Supergirl was never seen again after her twenty-first birthday, Exact Words may apply if that was all that they said and Braniac 5 merely assumed it meant that Supergirl would die in the future. They might not have said that Supergirl was never seen again after her 21st birthday and that only two members of the Justice League returned to the present because she chose to stay in the future, since it reminded her of her home and she had fallen in love with Braniac 5.