'It is [sic] can no longer simply be said that all those in any "irregular" situation are living in a state of mortal sin and are deprived of sanctifying grace. More is involved here than mere ignorance of the rule. A subject may know full well the rule, yet have great difficulty in understanding "its inherent values", or be in a concrete situation which does not allow him or her to act differently and decide otherwise without further sin.' (AL 301)
The theologians claim that this statement understood as meaning that "a person with full knowledge of a divine law can sin by choosing to obey that law", is heretical.
Taken as an ordinary person would take it, this is most certainly not heretical. Indeed, St. Thomas Aquinas and, after him, most moral theologians, recognize the case of someone obliged by their conscience to do something contrary to a divine law; this is in fact a classic case of a "(qualifiedly) perplexed conscience". (E.g., in de Veritate, q. 17, a. 14, objection 8 and response, Aquinas considers the case of someone whose conscience obliges him to commit fornication, and in Quodlibetal 3, q. 12, a. 2, of a heretic obliged in conscience to preach against the Catholic faith.) While some theologians of his time argued that such a person was not obliged to obey their conscience, St. Thomas Aquinas argues that such a person is obliged to obey their conscience, and sin if they do not, at and the same time, they sin if they do obey their conscience, by acting on an erroneous conscience that they should have corrected. Aquinas notes that such a person is not absolutely forced to sin, only conditionally, so long as they do not correct to conscience. Just as, so long as someone accepts a false principle as the basis of his thought, he cannot avoid error, so, so long as a person persists with an erring conscience, he cannot avoid sin. (ST I-II, q. 19, a. 6, ad 3) This teaching of St. Thomas Aquinas is taught by the Catechism of the Catholic Church: "A human being must always obey the certain judgment of his conscience. If he were deliberately to act against it, he would condemn himself." (CCC 1790)
One could say that the theologians' claim is technically correct, since, if a person's conscience tells him that he is obliged to do something incompatible with obeying a certain divine law, though he will sin if he disobeys his conscience and obeys that divine law, it would not be by choosing to obey that law that he sins, but by disobeying the law that his conscience puts forth as the reason why he is obliged to do something else. But, if our concern is with what Amoris Laetitia intends to say, or with how an average reader will take it, this technicality seems not to be really relevant. Under the given circumstance, if he chooses to obey that divine law, he will be sinning.