I don't think liking the Tridentine mass has any causal connection with thinking  that AL "propagates heresy"; but the Lefebvrists are marked not only by their liturgical tastes but also by their rejection of the teaching of Dignitatis Humanae, the Vatican II document on  religious liberty. And I would bet that the vast majority of the critics of AL have not profoundly assimilated this teaching of the Council.  At the basis of Dignitatis Humanae is a teaching about conscience that is very much tied to what AL teaches about conscience. DH teaches that  religion is essentially something inward; AL teaches that morality is essentially something inward, that it is not essentially about the merely external keeping of rules. At the base of both is a conception of conscience which is Biblical and ecclesiastical, but at the same time inward and personalist. The Church has been moving in this direction, and Pope Francis is struggling to keep us moving.