Why is denial of the primacy of the bishop of Rome considered a Schism, not a Heresy? Why is denial of the primacy of the bishop of Rome considered a Schism, not a Heresy?

Disclaimer: I'm interesting only in authoritative answer according to the Roman Catholic Doctrine.

The Papal primacy is essentially dogma of Roman Catholic Church. So why are those who deny this dogma (say, Orthodox Church officials) not formally considered as heretics?
According to Canon law, it is technically both:

Can. 751 Heresy is the obstinate denial or obstinate doubt after the reception of baptism of some truth which is to be believed by divine and Catholic faith; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Supreme Pontiff or of communion with the members of the Church subject to him. - Code of Canon Law

However, an arguable effect of JOINT CATHOLIC-ORTHODOX DECLARATION
OF HIS HOLINESS POPE PAUL VI
AND THE ECUMENICAL PATRIARCH ATHENAGORAS I is that it is no longer appropriate to affix the label &quot;heretic&quot; across the schismatic divide:

...in common agreement, declare that:
...
B. They likewise regret and remove both from memory and from the midst of the Church the sentences of excommunication which followed these events, the memory of which has influenced actions up to our day and has hindered closer relations in charity; and they commit these excommunications to oblivion. [emphasis added]