Union with Brahma, or company of Brahma?
In DN 13 the term is originally used by Brahmins talking with each other, and so between them it meant whatever (non-Buddhist doctrine) they each understood it to mean.

At the end of the sutta the Buddha teaches them the four brahmaviharas and defines or explains that these are the path to "companionship with Brahma" -- i.e. I think he thus redefines brahmasahabyatāya (whatever it used to mean) in Buddhist terms, in a way that can be understood and agreed by the Brahmins.