Some of what the article says is accurate as far as it goes which isn’t very far

. Yes, Leo is recognized by many historians as being the first Pope. But since the Pope is first and foremost the Bishop of Rome these historians are in serious error. Leo was not the first Bishop of Rome, St Peter was. Therefore since, St Peter was the first Bishop of Rome he was also the first Pope. Rome was always looked to by the other churches as having dominance and the leadership role. If a dispute arose among the Churches Rome settled it. The Bishop of Rome was accorded primacy by all of the other Bishops, Even the Orthodox today still see the Pope as first among equals and recognize his primacy but not his infallibility. Rome was considered to be special due to it being sanctified by the blood of two of the Apostles martyred there, Sts Peter and Paul, as well as the blood of those who perished in the persecutions.
Yes, prior to the great schism there was only one main church, the Catholic Church which was governed from… ROME and headed by the … POPE. There were scattered others here and there, Coptic for instance., but they were few and far between The great schism in which the Orthodox broke away came about in great part due to the eastern Bishops, in particular the Patriarch of Constantinople, wanting more autonomy. They felt that Rome had taken more of a leadership role than she was entitled to. This led to the ultimate excommunication of the Orthodox Churches. Simplified version of the events but essentially accurate. There were also some disagreements over the filioque clause of the Nicene creed and some minor doctrinal differences. However, the dispute was basically political in nature.
The protestant reformation which led to the establishment of the protestant churches was also in great part political with northern Europeans being frustrated with the governing of the Church by Rome. They felt that they were being effectively frozen out of important decisions. There were abuses in the Church at that time to be sure, almost off of which were corrected through the counter reformation, which all things considered should have brought the protestants back if that was really the problem, which unfortunately it wasn’t. It was an excuse.
The Protestants also took the position that scripture was to be interpreted by all to their own understanding without the need of a church interpretation. Sounds like a reasonable idea, however, at last count there were many thousands of protestant denominations, all separated by minor differences in interpretation of scripture, which kind of shows the folly of their position.