Maybe 'voice' has taken on a new meaning fairly recently, but to me it's nothing like 'opinion',

It's much like 'tone' & is determined partly if not mostly by the author's diction (choice of words), phrasing and the like & not related at all to opinion. Saying that a narrator has a strong voice is praise. Nothing to do with opinion. In fiction voice is probably most striking in 1st-person narratives.

Author's opinions are okay if the writer is so skilful that they aren't apparent. Animalia by Jean-Baptiste Del Amo is an unusually powerful book about a few generations of farming family that I never suspected was written as a protest against mistreatment of animals, but Ayn Rand and Scarlett Thomas don't write well enough to use their advocacy of selfishness or homeopathy as anything but blunt objects.