Semantics aside, the point is that the statute didn’t take anything out of the public domain that had already been put there.
As the majority opinion noted, there is a considerable history of Congress treating copyrights in force and prospective copyrights the same for purposes of term extensions. And it makes sense to do so, because doing so promotes certainty and definiteness. One of the benefits of a life plus X regime for copyright is that it provides a definite time from which the life of a copyright can be measured, and allows the whole corpus of an author’s work to pass into the public domain at the same time, regardless of date of creation. If we create a two-tiered regime where works created before date A are entitled to life plus X and works created after date A are entitled to life plus Y, we reintroduce the date of creation as a variable in determining copyright terms. And the whole body of an author’s work will not pass into the public domain at the same time. A two-tiered system reduces certainty and creates a fertile field for litigation, in the sense that the date of creation is much more likely to be a disputed topic than date of death.