In plain words
Even here, birth is conquered by those whose mind rests in equality. Brahman is flawless and equal; therefore they abide in Brahman.
Word for word
Here itself birth is conquered by those whose mind is established in sameness; for Brahman is flawless and the same; therefore they are established in Brahman.
The commentators
Bhagavadgītābhāṣyam
ननु अभोज्यान्नाः ते दोषवन्तः, ‘समासमाभ्यां विषमसमे पूजातः’(गौ. ध. २ । ८ । २० ; १७ । १८) इति स्मृतेः । न ते दोषवन्तः । कथम् ? —
इह एव जीवद्भिरेव तैः समदर्शिभिः पण्डितैः जितः वशीकृतः सर्गः जन्म, येषां साम्ये सर्वभूतेषु ब्रह्मणि समभावे स्थितं निश्चलीभूतं मनः अन्तःकरणम् । निर्दोषं यद्यपि दोषवत्सु श्वपाकादिषु मूढैः तद्दोषैः दोषवत् इव विभाव्यते, तथापि तद्दोषैः अस्पृष्टम् इति निर्दोषं दोषवर्जितं हि यस्मात् ; नापि स्वगुणभेदभिन्नम् , निर्गुणत्वात् चैतन्यस्य । वक्ष्यति च भगवान् इच्छादीनां क्षेत्रधर्मत्वम् , ‘अनादित्वान्निर्गुणत्वात्’(भ. गी. १३ । ३१) इति च । नापि अन्त्या विशेषाः आत्मनो भेदकाः सन्ति, प्रतिशरीरं तेषां सत्त्वे प्रमाणानुपपत्तेः । अतः समं ब्रह्म एकं च । तस्मात् ब्रह्मणि एव ते स्थिताः । तस्मात् न दोषगन्धमात्रमपि तान् स्पृशति, देहादिसङ्घातात्मदर्शनाभिमानाभावात् तेषाम् । देहादिसङ्घातात्मदर्शनाभिमानवद्विषयं तु तत् सूत्रम् ‘समासमाभ्यां विषमसमे पूजातः’(गौ. ध. २ । ८ । २०) इति, पूजाविषयत्वेन विशेषणात् । दृश्यते हि ब्रह्मवित् षडङ्गवित् चतुर्वेदवित् इति पूजादानादौ गुणविशेषसम्बन्धः कारणम् । ब्रह्म तु सर्वगुणदोषसम्बन्धवर्जितमित्यतः ‘ब्रह्मणि ते स्थिताः’ इति युक्तम् । कर्मविषयं च ‘समासमाभ्याम्’(गौ. ध. २ । ८ । २०) इत्यादि । इदं तु सर्वकर्मसंन्यासविषयं प्रस्तुतम् , ‘सर्वकर्माणि मनसा’(भ. गी. ५ । १३) इत्यारभ्य अध्यायपरिसमाप्तेः ॥ १९ ॥
In plain words
Objection: then their food may not be eaten; they are tainted — for the smṛti says, "because he honours the unequal as an equal and the equal as an unequal" (Gautama Dharma-sūtra 2.8.20; 17.18). No. They are not tainted. How so? —
Right here, in this very life, birth itself is already conquered by those wise ones who see with equal eyes — for them the mind rests, unwavering, in the equality of all beings, in Brahman. 'Faultless': though it may look, to a deluded eye, tainted by the faults of outcastes and the like, Brahman is never actually touched by any of it — which is exactly why it's called faultless, utterly free of fault, because that's simply what it is. And it isn't split up by differences of its own qualities either, because consciousness itself has no qualities to begin with. Krishna will later say as much: desire and its kin belong to the field, the body-mind, not to the Self — 'because it's beginningless, because it's without qualities.' Nor are there, deep down, any final marks that could actually divide the Self body by body — no proof could ever establish such marks exist. So Brahman is one, and equal everywhere. And that's why these wise ones are established in Brahman alone, and why not even the faintest whiff of fault ever touches them — because they've simply stopped mistaking the body-heap for the self. There's another old rule that talks about treating the equal and unequal differently 'in matters of worship' — but that only applies to someone who still makes that mistake about the body; it's specifically about worship. You see this in practice: 'he's a knower of Brahman,' 'he knows the six sciences,' 'he knows all four Vedas' — such distinctions decide who gets worship and gifts. But Brahman itself has no connection to any quality or fault at all — which is exactly why it's right to say 'they are established in Brahman.' That rule about treating unequal and equal differently belongs to the world of ritual action; but what's under discussion here, from 'having renounced all actions with the mind' all the way to the end of this chapter, is the renunciation of all action altogether — a different matter entirely.
Close to the Sanskrit
But surely they are ones whose food is not to be eaten, they are possessed of fault — because of the smṛti, 'on account of honouring the unequal and the equal with equal and unequal (honour)' (Gautama Dharmasūtra 2.8.20; 17.18). No, they are not possessed of fault. How? —
'Here, even while [still] living,' by those wise ones who see equally, 'is birth conquered,' [i.e.] brought under control — for whom the mind, [i.e.] the inner faculty, 'is established in the equality among all beings, in Brahman,' [i.e.] in the state of equality, become unwavering. 'Faultless' — although among the faulty, outcastes and the like, it is imagined by the deluded, by their faults, to be as if faulty, nevertheless it is untouched by those faults; hence [it is called] 'faultless,' [i.e.] devoid of fault, since it is so. Nor is it differentiated by any distinction of its own qualities, since consciousness itself is without qualities. And the Lord will state that desire and the rest are attributes of the field, not the Self: 'because of beginninglessness, because of being without qualities' (Gītā 13.31). Nor, again, are there any final distinguishing particulars that differentiate the Self, since, for each body, the existence of such particulars cannot be proven. Therefore Brahman is equal and one. Therefore they [the equal-seeing wise] are established in Brahman alone. Therefore not even the faintest trace of fault touches them, since they lack the false identification of the self with the body-aggregate. But that other aphorism, 'by the equal and unequal, toward the unequal and the equal, in matters of worship' (Gautama Dharmasūtra 2.8.20), applies only to one who still has that false identification with the body, since it is qualified as being about worship. For it is seen that ['this one is a] knower of Brahman,' '[this one is a] knower of the six auxiliary sciences,' '[this one is a] knower of the four Vedas' — the connection with a particular quality is the reason in [determining] worship, gifts, and the like. Brahman, however, is devoid of connection with any quality or fault whatsoever; hence, 'they are established in Brahman' is correct. And that other aphorism, 'by the equal and unequal...' concerns [ritual] action, whereas this [Gītā verse] is presently under discussion concerns the renunciation of all action, beginning from 'having renounced all actions with the mind' (Gītā 5.13) up to the end of the chapter.