Perhaps you did not watch carefully...the couple was told (erroneously) by a Princess rep that they did NOT need a passport and that BC was OK. So...they went with only a BC and were of course, denied boarding.
I generally do not watch the videos from the channel that I posted...I could not quickly find the one I wanted from Don Terris:
https://www.youtube.com/watch?v=GD_NNhcuXNE&t=6s
Perhaps this one is a bit more clear as to what happened. It's ALL about trying to use a birth certificate. (The first one was, too.)