That’s an interesting situation, and honestly, I don’t think getting back with an ex is forbidden—it just depends on the people involved. My cousin actually remarried his ex-wife after three years apart. At first, he thought their issues were too big to fix, but with time, they both grew as individuals.
Divorce in this case was like a truce period in wars before moving on to negotiations that will achieve peace. It is a good stage to stop the conflict without completely severing relations, and in the meantime, negotiations can succeed in achieving a lasting peace based on the satisfaction of both parties. The problems between them will most likely be solvable, but they did not know the best way to manage their crises in a period of psychological pressure imposed by the responsibilities of the relationship. Also, their success in rebuilding the relationship means that one of them needs the other and that their relationship is complementary. Most likely, their relationship has returned to become stronger and more solid than it was before the divorce, and they certainly have become more able to manage their crises and control everything.