Swami Sivananda has translated the Brahmasutras into English, "based" on Sankaracharya's bhashya, available online at http://www.swami-krishnananda.org/bs_0.html. The particular interpolation comment can be found at http://www.swami-krishnananda.org/bs_1/1-3-09.html, where no reason is given. Chronologically, he is the fourth one to translate Sankara's bhashya into English after Rev. K. M. Banerjea (a Christian apologist who says that Jesus is the Vedic Prajapati), George Thibaut and Swami Vireswarananda. The distinguishing aspects of Swami Sivananda's translation are:
a) He edits the material so as to focus on what is relevant to the busy man of 1940s.
b) For translating the various Vedic sentences quoted within the bhashya, he relies on Thibaut - sometimes even when Thibaut is patently incorrect. For instance the sentence "तदेतद् ब्रह्म अपूर्वम् अनपरम् अनन्तरम् अबाह्यम्" Br. Up. 2.5.9, Thibaut translates it as "This is the Brahman without cause and without effect, without anything inside or outside; this Self is Brahman perceiving everything". Swamiji uses the exact same translation. However, as per the commentary of Sankara on the Brihadaranyakopanishad, Brahman is not without anything inside; here anantaram means homogenous, i.e. without any divisions within. If Brahman were without anything inside, how would It be different from Sunya? Thibaut used the translation he did, because he was following the translation of his General Editor, Max Muller. Max Muller was not under any obligation to follow Sankara's views on what the Upanishad sentences mean, but Thibaut himself states in the preface to his translation of Sribhashya, that a translator should translate the Upanishad sentences in line with the views of the original author. So, Thibaut was obliged to follow the views of Sankara while translating Sankara's works, and yet did not. However, his translations were carried over by other translators (not merely Swamiji, but also by V. H. Date and V. M. Apte to varying degrees). In Swami Gambhirananda's works, we find an independent (of Thibaut and Max Muller) translation.
The point is that Swamiji focussed on making the bhashya easily intelligible to the common man and not on critical or scholarly evaluation. Thus, I doubt his comments are based on looking at manuscripts. Further, he does not cite any manuscripts in support of his statement, which itself is speculative in nature: "The whole of this Adhikarana about Sudras together with the preceding one about the Devas appears to be an interpolation of some later author." (emphasis supplied).