The question of whether or not Dogen ever taught "practice and enlightenment are one" or "continuous practice, continuous enlightenment" is not important in the universe. Yet, it is also more than merely an intellectual exercise.
They are teachings that are certainly helpful and a path of liberation for at least some students. However, if they were not taught as believed and claimed by Soto folks, or are publicized falsely to be a mistaken interpretation or "misnomer," then at least some of those students may come to believe that they are not correct teachings or merely in error, and they might not undertake that path or lose dedication believing the same to be in error. So, to clarify history and whether or not something is a mistake or misnomer is important in that sense. One can teach something even if the history is mistaken of course (Zen and other Buddhists, as well as all religions, do this all the time with many of our ahistorical beliefs and claims, and many students are fine with myth and benefit from that), but it is surely good to trace the origins and tradition behind some claim about our practice when that history is relatively clear. Dan has it exactly right ...
But, better than a lie, it is a good to teach them a demonstrable truth to get them out of the house when one has such a truth. Those who wish to believe the myths instead can and will. Those who will believe neither, and will stay in the burning house, can and will too.... it boils down to the simple question of skillful means. What helps best? A lie about golden leaves to entice the kids out of the burning house or just tell them there is no burning house and most likely watch them writhe in agony? We know even delusions are empty and non-existent and yet..
In this case, it has been possible for so many scholars and others to demonstrate through his many writings that Dogen taught "practice and enlightenment are one" and "continuous practice, continuous enlightenment," so it is simply good to make that history clear and not let incorrect statements stand without some challenge. If someone presents a bit of personal scholarship and historical research as did Greg, then they should be prepared for polite challenge to their assertions to see if they hold water. One can respect the person, yet challenge their proposal. That is how historical and other research advances.
We live in a world where religions so often are based on all manner of wild and ahistorical claims, so it is nice to be able to say to prospective and current students, "in this case, it actually is true."