When the 2 previous popes taught truths on any subject, they always reaffirmed the other truths about that subject, and they were careful not to leave any loose ends that cunning people would use to undermine other parts of Catholic doctrine. If they explained one aspect about one sacrament, (like Marriage) they maintained consistency with other aspects of that sacrament, and left no loopholes liberals could use to weaken teaching on other sacraments (like Eucharist or Confession).
You are right, he is not contradicting essential doctrine, but leaves the door open for others to do so.
The 2 prior popes rarely spoke on doctrinally related issues extemporaneously. I think when they wrote something, they apparently had it vetted by someone, with this assignment: “Given the blind spots and biases dominant in today’s world, what is here that today’s knaves, fools, or the misinformed might be inclined to misinterpret, take out of context, etc?” Then their work was properly edited before publication.
In other words the 2 prior popes were far more aware of the secular world, and its biasing or blinding capabilities, and they took that into account.
(It was hard for me to write that, after opposing his critics for 4 years in person and online).