Bhashya
ब्रह्म अपरम्, सर्वभावस्य साध्यत्वोपपत्तेः ; न हि परस्य ब्रह्मणः सर्वभावापत्तिर्विज्ञानसाध्या ; विज्ञानसाध्या च सर्वभावापत्तिमां— 'तस्मात्तत्सर्वमभवत्' इति ; तस्माद्ब्रह्म वा इदमग्र आसीदित्यपरं ब्रह्मेह भवितुमर्हति ॥
In plain words
It is the lower Brahman that is meant here, because attaining the state of all is something that can be accomplished through means. For the supreme Brahman, the state of becoming all cannot be brought about by knowledge, whereas the text says 'Therefore that became all,' showing that this state is accomplished by knowledge. Hence, the statement 'Brahman indeed this in the beginning was' must be understood as referring to the lower Brahman.
Close to the Sanskrit
Lower Brahman, because the state of being all is capable of being accomplished; for of the supreme Brahman the state of becoming all is not accomplished by knowledge; and the state of becoming all is accomplished by knowledge—'Therefore that became all' thus; therefore 'Brahman indeed this in the beginning was'—the lower Brahman here is appropriate to be.