we don't know for sure if it was retconed
zoro's breath is based on the concept of ryou flow
that was not just basic armament, normal armament is very simple you just have to force your will, while the flow is more technical than that. luffy said it when he was training for ACoA
zoro said it's a power to cut steel is the ability to hear the breath, he would only use it when he needs to cut very tough things like (daz bones, the sea train, kaku's tekkai, kuma) it was a complicated process that zoro could only use it in near death situations or after deep concentration
zoro was in a near death experience and got a haki bloom, made him able to flow both armament and observation at the same time
we leaned recently that you can coat things with CoC just like CoA, it's completely possible that you can coat yourself with CoO too and zoro did that that's why he was able to hear the breath of all things