San Jose Bishop McGrath and Los Angeles Roger Cardinal Mahony have both taught heresy, whether you want to call it informal, material or formal doesn't matter.
Aquinas taught that not only can bishops be heretics, but they lose jurisdiction when they do.
You certainly like to cut and paste. The problem is, you pull citations from anywhere out of context in an attempt to prove your point. The piece you quoted speaks to bishops who keep the Faith. It does not say it is impossible for a bishop to fall into heresy.
If His Excellency Bishop McGrath and His Eminence Cardinal Mahoney have in fact pertinaciously believed something contrary to a truth of divine and catholic faith, they have fallen from their sees and are no longer Catholic bishops (cf. c. 194). However, there will always be at least some true Catholic bishops.