It is in Unam sanctam.
papalencyclicals.net/Bon08/B8unam.htm
As far as an explanation I will let someone else try to do that. It seems to me that if Quo Primum can be tossed aside, then so can this. But I say that merely tongue in cheek as it seems that there is a cafeteria mentality when it comes to encylicals and apostolic constitutions.
However, I will add that at the time it was written, there was ONLY the catholic church. Protestantism wouldn’t begin for another 200 years or so, and so one must look that if there was only one faith, that this must apply to all those who are part of that faith. i.e.Catholics under the jurisdiction of Rome. However, that then opens up a can of worms again when looked in light of how other papal decrees have been put aside. If what was written by Pope Boniface was the truth, and once a truth always the truth, then how do we then reconcile that with the ignoring of the Syllabus of Errors, or Quo Primum.