– God has not taken a son, nor is there any (other) god with Him
This verse seems to connect the concept that Allah has taken a son with the idea of a separate second god. Verse 92 links it further with associating (shirk) someone with Allah. See a summary of verses that accuse the People of the Book of “associating” at 61.9.
The context of this denial of Son of God does not suggest that the Quran is addressing Christians. However, the logic of verse 91 is worth noting. If Allah had a son, according to this verse, the result would be two gods; the two gods would then compete and exalt themselves over each other.
Among the three persons of the one God of the New Testament, however, there is no rivalry but rather mutual submission and a fellowship of love.