p. 258
आख्यानं पुराणं पञ्चमं येषां वेदानां ते आख्यानपञ्चमाः ।श्रूयते च्छान्दोग्ये—‘इतिहासपुराणं पञ्चमम्’ इति ।तैः आख्यानपञ्चमैः वेदैः भूयिष्ठम् अत्यर्थं कत्थ्यते श्लाघ्यते
In plain words
Narrative — that is, the Purāṇa — is the fifth of these Vedas; hence they are called ākhyānapañcamāḥ, 'the ones whose fifth is story.' The Chāndogya says as much: 'the Itihāsa-Purāṇa, the fifth.' By those Vedas, story-as-the-fifth included, He is praised bhūyiṣṭham — beyond all measure. Praised: extolled.
Close to the Sanskrit
Narrative, the Purāṇa, is the fifth of which Vedas — those are the ākhyānapañcamāḥ, 'those whose fifth is narrative.' And it is heard in the Chāndogya: 'the Itihāsa-Purāṇa as the fifth' — thus. By those Vedas whose fifth is narrative, He is bhūyiṣṭham, exceedingly, katthyate, extolled.