Okay, there's a problem with this whole lesson. It is correct, but it's not used in the way it shows here. It is not used like this, especially when saying that you're going to do simple stuff. You will never hear a Greek person say "Πρόκειται να πω την αλήθεια". What we use is "Θα πω την αλήθεια". In situations like this one, for instance, "I'm going to go to the store" (πρόκειται να πάω στο μαγαζί, if translated directly), you wouldn't use πρόκειται να, you would use simple present. "Θα πάω στο μαγαζί" (I'm going to go to the store) "Θα πω την αλήθεια" (I'm going to tell the truth). It is correct, but it is not used that way.