The Catechism (paragraph 1650) does not distinguish whether the sin is mortal, but that the situation objectively contravenes God’s law. It isnt concerned with the culpability of the couple, only that the situation is wrong. As long as the couple lives in complete continence, the situation is resolved. This is the manner which the doctrine removes the couple from the barrier of adultery. Saying it wasnt mortal sin for them to get into the situation does not mean they can embrace the adultery.
“In other words, if the prior marriage of two divorced and remarried members of the faithful was valid, under no circumstances can their new union be considered lawful and therefore reception of the sacraments is intrinsically impossible. The conscience of the individual is bound to this norm without exception.[2]”
This is what two previous popes and the Church affirmed in a Magisterial manner. As for now, I hold those pronouncements more authoritatively than AL. We will see if the AL suggestion holds ground. I have a feeling it will not prevail.