It indeed would have been heresy, but no one said that it is. It is among the five non-negotiable principles of social policy, as opposed to other aspects of social policy where reasonable Catholics can take diverging views, but not any kind of highest Church dogma.
"“No, the taking of innocent human life is so heinous, so horribly evil, and so absolutely opposite to the law of Almighty God that abortion must take precedence over every other issue. I repeat. It is the single most important issue confronting not only Catholics, but the entire electorate.”"
That's rank heresy, but his violation of the canons relating to his episcopal jurisdiction should be enough. The man should be silenced, deposed and sent to a monastery.