Who told you that?? I suppose, in strict primary/secondary hierarchies, there could be an agreement that "We share everything about our other relationships in order to make sure nothing is threatening the primary relationship" but that would leave the secondaries with very little privacy and could be uncomfortable for every involved at times!
My husband and my partner hear a LOT about my relationship with the other, the good and the bad, because they are also both my closest friends. They both know this and are okay with it. However, I do NOT discuss intimate details of one with the other. It's not their business, and would likely make them both very uncomfortable.
Many folks around here share even less across relationships. Honesty for the sake of sexual health is one thing, but beyond that there's a strong sense of respecting everyone's privacy and independence. Wanting to know ALL the details of another relationship is seen as nosy and intrusive, and perhaps as a sign of being insecure in one's own relationship(s).
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack