Didn't have to go that far back for the narrative, went there because: that seems to be the place one can find an exposition not couched in mealy mouthed words, where an explanation isn't necessary, where no translation is necessary. I don't agree with him, but I can understand him.
You also need to understand the context of the time. The Church was a waning political force. He was attempting to reclaim some of that stature, as well as defend its independence from outside interference.
He was a highly trained diplomat, as well as a learned theologian. However, he had a bit of an ego, according to reports.
Part of the goal in that particular Bull was to draw a line in the sand. He was also calling on the implied fear of excommunication for political leaders. Sadly, this is wildly inappropriate, regardless of the reason.
Up until really the 20th century, there has been a struggle to protect the independence of the Church from physical encroachment of world powers, while fulfilling its mission to spread the Gospel. Some of the previous Popes took the political side more seriously, and vice versa. However, the fear was real, as foreign powers were regularly sticking their nose into Church affairs, and making trouble for the faithful.
Some people think we have it bad in the US vis a vis govt. interference with religion, especially Obamacare. While it is true, nothing really compares to early middle ages through Henry VIII.