"Steam does not contain any paid advertising, nor are advertising models supported in games distributed on Steam. There are varying interpretations for what constitutes ‘advertising’ in a game, so the examples below are meant to help guide developers as to what is and is not supported on Steam."
Under not supported: "Developers should not utilize paid advertising as a business model in their game"
The Borderlands 4 adverts in BL3 are not paid adverts, and they don't advertise anything other than their own games.