Magnus is great, but he has not had a long track record yet. and he hasn't written any books. Gary is still the greatest and Magnus may one day overcome that accolade. but not now.
How long a track record does he need? He's been actively +2800 longer than any player in history, including Kasparov! Kasparov was +2800 from 1997-2005 when he retired. Carlsen has been +2800 from 2009-2018, and he's still active.
Kasparov's modern record for longevity as World Champion isn't under threat yet, (1985-2000, with five successful title defenses) but Carlsen is closing the gap even there. Carlsen won the World Championship in 2013, and has since successfully defended it twice.
But in terms of longevity, Carlsen has already proven himself to be the real deal, and has maintained a +2800 rating longer than any player in history. Oh, and he has the highest rating in history.
80 more minutes!