Hmm. I think there’s a difference between omniscient of that kind, which might be a kind of unseen and unvoiced narrator, and one who directly addresses the reader of the “As you know, magic is but a figment of the imagination” kind which is what I think is meant here.
Omniscient is fine, if not universally applauded any more, but to my mind directly addressing the reader needs to be handled very carefully. It works with old-fashioned children’s stories, and with comic writing, and it could work as an extended piece in first person written eg as a letter to a reader or as a warning for whoever may come after. But in an otherwise third person serious narrative, it would likely pull me out of the story and make me question the writer’s technique and ability. My advice in those circumstances, for someone who is relatively new to writing, is to avoid it.