He was in the NO, but where’s your evidence for the charge that he was a heretic?
I'm sure if you ask him he will agree - if not now, just give him a little more time and he should.
Simply, the NO is a modernist religion, modernism is the synthesis of all heresies, he was a NO Modernist bishop= he was a NO heretic bishop. I am not differentiating material/formal in my comment because now he is back, so it really does not matter.