One of my personal interpretations was that this brahma was actually compassionate part of the mind of the Buddha (something like powerful mindstate of loving-kindness, a brahma-vihara). Maybe it shows that reaching enlightenment gives you this feeling of “leaving the world of suffering”, and just chilling in very high or nibbanic jhanas and general arahanthood, and that it is so great that you no longer want to get “your hands dirty” in the world with all its ups and downs, disrespectful people, possible dangers etc.
But the Heart aspect of compassion was stronger, so Buddha went teaching even when he could just chill in the nibbanic jhanas to the end of his days?
My interpretation is that this sutta shows that going teaching out of compassion was actually a CHOICE and not mandatory, showing that enlightened beings are not a compassionate-machine that is driven by some unstoppable force of Love, but being that can still make a choice, and chooses out compassion and love to help others.
Another importaint aspect in my opinion is that, it shows that at least some brahmas (if not most) are actually on the side of dhamma, and they want good of people, even if it means reaching Nibbana.
It is quite different from the brahma that Buddha met in Brahmanimantanika sutta, which Buddha had to convince that dhamma and Nibbana are better than being in brahmic plane.
Also it shows that this Brahma Sahampati is completely different than deva Mara, who wish for people to stay in samsara, and wanted that Buddha did not teach.
So, it shows that some Brahmas are friends of Dhamma (unlike some Devas who can be very deluded and want just to explore more sensual pleasures) and can be trusted to help us on the Path, if we have minds that can communicate with them.
So, Brahmas are friends of Buddhas if they only see the truth of Dhamma I think it is importaint to remember that they are on the side of good, and I believe it is part of messege of this sutta.
But that’s just my interpretations
I’m very curious about others take on this.