Invention of the 4th-person narrator - Fates and Furies

I'm only 2 chapters into Lauren Groff's much-lauded Fates and Furies so not ready to judge yet but I want to tip my figurative hat to her for her narrative innovation. Maybe others have done things like this before - obviously, writers such as Nabokov are known for writing "commentary" on their own narratives, but Groff seems to have invented a narrative technique that I'll call the 4th-person narrator: the narrator (traditional omniscient 3rd person) tells the story - which in these first two chapters is the back story, back a whole generation in fact, of the 20-something newlywed husband we meet on the first page having sex with his new bride on a rocky beach on the coast of Maine - but at various moments she has a higher-level narrator - something like the author verifying or clarifying points in her own narration, but not quite the author, either, it's just another layer of narrator, even more omniscient. Sometimes these interruptions are for a few words, sometimes a paragraph or longer - and they all have the purpose I think of making the fiction seem to be (though it's not) real, factual - it's told by a narrator and verified or clarified by a super-narrator. I admire this because in the very first writing class I ever took - sophomore year and JHU many years back - the prof challenged us all, in passing, to invent a 4th-person narration. I tried to rise to the challenge, wrote a story in which, at the end, the narrator reflects back on the story and its significance. The story is long lost but to my memory it was a pretty good story but the authorial reflection, while a good concept, was handled in a juvenile manner (what the hell, I was 18 years old). I wish I'd been as smart as Groff and incorporated that narrative voice into the story, braided it in, rather than stamping it on as an appendage. Anyway, a promising start, at least in some ways - the narrator, both of them perhaps (we may learn that the husband and wife are commenting on each other's stories?), seems to have us in good hands.