Wrong. You must believe someone is pope before you can be schismatic. If you truly believe someone is not the pope, but affirm papal authority nevertheless, there is no schismatic intent.
In the early fifteenth century there were once three claimants to the papal office and each had his adherents. Were those who denied the authority of the other two claimants schismatic? No.
According to the Catholic Encyclopedia, "under these circumstances good faith may, at least for a time, prevent a formal schism; this begins when the legitimacy of one of the pontiffs becomes so evident as to render adhesion to a rival inexcusable."
So good faith is the key. So also is the self-evidence of the legitimacy of the pontiff himself. I personally believe Derksen has been rash--but he seems to me in good faith nevertheless, even as the heterodoxy of this Pope has become more and more obvious.
This is a very poor analogy.
The antipopes of this time all claimed legitimate election by different factions of the College of Cardinals. They had, weak or strong, juridical claims.
Derksen is saying that Pope John Paul II was indeed elected by the College of Cardinals, but that by promulgating a Code of Canon Law that Derksen personally dislikes he invalidated his own election.
This is a novel claim and not at all analogous to the factionalism of the 1400s.
The Orthodox do not believe anyone is Pope or such a thing as a Papal office exists.
Yet they are schismatics without naming any alternate candidate.
A schismatic is a person who advocates sundering the unity of the Church.