The second reason is that Chandalas, as we know them today, didn't even exist in Vedic times. If the verse is authentic, it can not be referring to the group of people called 'Chandalas' today.
The above argument seems to be that, as chhAndogya upanishad verse 5.10.7 mentions the term "chandala", the verse cannot be authentic.
However, even the Brihadaranyaka upanishad, which also belongs to the same period as chhAndogya upanishad, mentions the term "chandala", in a verse referring to the state of deep sleep and/or brahman.
atra pitāpitā bhavati, mātāmātā, lokā alokāḥ, devā adevāḥ, vedā avedāḥ । atra steno'steno bhavati, bhrūṇahābhrūṇahā, cāṇḍālo'caṇḍālaḥ, paulkaso'paulkasaḥ, śramaṇo'śramaṇaḥ,; tāpaso'tāpasaḥ, ananvāgataṃ puṇyenānanvāgataṃ pāpena, tīrṇo hi tadā sarvāñchokānhṛdayasya bhavati || 22 ||
- In this state a father is no father, a mother no mother, the worlds no worlds, the gods no gods, the Vedas no Vedas. In this state a thief is no thief, the killer of a noble Brāhmaṇa no killer, a Caṇḍāla no Caṇḍāla, a Pulkasa no Pulkasa, a monk no monk, a hermit no hermit. (This form of his) is untouched by good work and untouched by evil work, for he is then beyond all the woes of his heart (intellect).
Hence, this is not sufficient evidence to argue that the chhAndogya verse is not authentic.
It is also to be understood from the above Brihadaranyaka verse,that in the state of brahman and deep sleep, all caste and status distinctions, relations, merits, demerits etc disappear.
Bhagavad Gita also says the same thing -
vidyāvinayasaṅpannē brāhmaṇē gavi hastini|
śuni caiva śvapākē ca paṇḍitāḥ samadarśinaḥ৷৷5.18৷৷
5.18 The learned ones look with equanimity on a Brahmana endowed with learning and humility, a cow, an elephant and even a dog as well as an eater of dog's meat.