This study is for household transmission in an extended setting. That's not the same as transmission e.g. in an outdoor environment or for short, casual contact.
Vaccinated people are less likely to be infected, hence "breakthrough case." Your quote is like saying that "people thrown out of windows in car crashes have the same rate of injury, despite whether the person was wearing a seatbelt or not." That may be true, but wearing a seatbelt definitely reduces the chance of getting thrown out of a window.
Vaccinated people are less likely to be infected, hence "breakthrough case." Your quote is like saying that "people thrown out of windows in car crashes have the same rate of injury, despite whether the person was wearing a seatbelt or not." That may be true, but wearing a seatbelt definitely reduces the chance of getting thrown out of a window.