Verse 78-80 of Skanda Purana is as follows:
Verse 78-80: He was born of Devaki and Vasudeva with Brahmagarga as leader (priest, preceptor). It was in the twenty-first Dvapara towards its close. When Dharma was spoiled Visnu was born in the Vrsni dynasty. He was born in order to establish Dharma. He destroyed the Asuras.
There are two points which I have highlighted, one is, it says son of Devaki and Vasudeva that is Lord Krishna as mentioned in many scriptures including Mahabharata, Adi Parva: SECTION LXIII.
And Vishnu himself, of world-wide fame, and worshipped of all the worlds, was born of Devaki through Vasudeva, for the benefit of the three worlds.
And the second highlighted thing is that that Skanda Purana verse is saying that Vishnu is born in Vrishni dynasty. Lord Krishna is of Vrishni dynasty as mentioned in Shrimad Bhagavaan, Canto 1, Chapter 3, Verse 23.
vṛṣṇiṣu prāpya janmanī
rāma-kṛṣṇāv iti bhuvo
bhagavān aharad bharam
In the nineteenth and twentieth incarnations, the Lord advented Himself as Lord Balarāma and Lord Kṛṣṇa in the family of Vṛṣṇi [the Yadu dynasty], and by so doing He removed the burden of the world.
So, that verse of Skanda Purana is talking about Lord Krishna but the only confusion is it says in 21st Dvapara while I used to know it as 28th Dvapara. I am not sure of it.