There is no discussion of asterism in this hymn. Why are you trying to see asterisms here?The reference to the herdsman in the hymn is to Surya. The cattle with go out with the rise of sun, then part from each other while grazing, make the return in the evening with the bells around their neck tinkling and rest in the night. That is what we know as 'Go-dhuli' (dust raised by cattle) in India. The cycle depended on the sun. Sun is the herdsman.
"I call upon their herdsman, him who knoweth well their coming nigh,
Their parting and their home-return, and watcheth their approach and rest."
The Home-leader is Indra, the other deities mentioned are Agni and Soma. All other deities are mentioned collectively (May all the Holy Deities pour down on us a flood of wealth)
But we are discussing why a particular asterism was named as "Punarvasu?
The reason must have been related to dwelling and riches. That is what I am pointing to, and not to the worship of this deity or that.
You agree that Punarvasu means 'dwelling/riches again'.
Just as mention of Ganapati in Rig Veda does not mean the Elephant-headed God we have now, Punarvasu is not asterism in this hymn.