He is referring to Quran 23:5-7 [1] and Quran 70:29-30. He did not mention this about concubines, because Quran 47:4 puts restriction on making of new slaves and concubines but for current slaves and concubines, a different strategy was adopted and their current status was not altered by Quran.

The above statement was in comparison to allowance for Mutaa created by some scholars, which is a different thing altogether.

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[1].

وَالَّذِیۡنَ ہُمۡ لِفُرُوۡجِہِمۡ حٰفِظُوۡنَ.اِلَّا عَلٰۤی اَزۡوَاجِہِمۡ اَوۡ مَا مَلَکَتۡ اَیۡمَانُہُمۡ فَاِنَّہُمۡ غَیۡرُ مَلُوۡمِیۡنَ.فَمَنِ ابۡتَغٰی وَرَآءَ ذٰلِکَ فَاُولٰٓئِکَ ہُمُ الۡعٰدُوۡنَ

And who guard their private parts except from their wives and slave-women because for these they have no blame on them. But, those who want something beyond this are indeed transgressors. (Quran 23:5-7)