We have explained in our introduction that all the commentaries in the market, they are simply presentation of the particular commentator's personal view. That is not Bhagavad-gītā. If you want to understand Bhagavad-gītā, then you should understand as they are said. You don't interpret in your own way.
Now, because there should be some doubt of the ordinary man, that "How Kṛṣṇa could say to the sun-god?" that is explained in the next verse. Because Arjuna was taking instruction from Kṛṣṇa, he knew Kṛṣṇa, what He is. Otherwise he would not have accepted him as a spiritual master. But because others would doubt, "This is fictitious that Kṛṣṇa said to the sun-god. How it is possible?" so you will find Arjuna said, "The sun-god Vivasvān is senior by birth to You. How am I to understand that in the beginning You instructed this science to him?" Kṛṣṇa is taking our position, er, Arjuna. Persons who are thinking of Kṛṣṇa as ordinary person, so Arjuna is trying to clear that point, that Kṛṣṇa is not ordinary person. He is the Supreme Personality of Godhead. Therefore he has put this question, that "The sun-god Vivasvān is senior by birth to you."