All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If you are referring to fatwa 355954, then the statement we made in it is the following: "As far as we know, there is no authentic evidence that the slave women used to walk around bare-breasted..." What we meant is that we could not find any evidence indicating that this used to happen during the time of legislation (establishing the Shariah), which is the lifetime of the Prophet, sallallaahu ‘alayhi wa sallam, because this would decide whether such an action is permissible or not. This does not contradict the fact that it happened during the time of the generation that followed the Taabi'is (generation following that of the Companions), in which Imam Maalik lived.
We do not know what you mean by asking whether the narration you cited in your question is true. If you mean the authenticity of attributing it to Imam Maalik, then it is authentically attributed to him. It was also cited in Mawaahib Al-Jaleel by Al-Hattaab, who attributed it to Ashhab's listening to Maalik. As you can see, Imam Maalik denounced this action and raised the matter to the Sultan to forbid them because of what this involves of corruption. This means that the claim that it is permissible cannot be ascribed to the Shariah.
We have previously underlined in the fatwa which you referred to in your question that investigating such issues is of no benefit because it does not lead to doing good deeds since slavery does not exist in our present time. A Muslim should, rather, preoccupy himself with what benefits him in his religion and his worldly life.
Allah knows best.