Here is what Śaṅkara says on this topic in his bhāṣya on Bṛhadāraṇyaka Upaniṣad 1.4.7. Following this, I have added some observations on what he says (Swami Madhavananda translation).

Objection: The topic was knowledge – when the Self is known, everything else is known. So why is a different topic, viz. attainment, introduced here?

Reply: Not so, for the shruti uses the words ‘knowledge’ and ‘attainment’ as synonymous. The non-attainment of the Self is but the ignorance of it. Hence the knowledge of the Self is Its attainment.