Let's wait until the end of May but maybe we're already at that point with go as well.
(We don't know yet if I can win a running race against a Ferrari, still I'm pretty sure about the outcome.)
Plus, he can play against an engine any time he wants. If he could beat them in some magically surprising way, would it make any sense for him to be keeping this amazing skill a secret from the world? No.
Sources?
https://www.google.com.au/search?q=chess+engine+elo+rating
And we don't know "with 100% certainty", but we can calculate a projected win/loss probability using those ELO ratings[1]
A 3300 rated chess engine vs Carlsen at 2850 gives a 93% win probability to the chess computer.
That's roughly the same probability of an International Master raned player (ELO 2300) beating Carlsen.
The top ranked engine is Stockfish, rated 3500+
Just because it was not calibrated with humans does not mean it is not an "Elo" system.
As far as Im aware no engines compete against humans regularly enough to get an ELO score (presumably because watching a computer thrash human players for a few hundred games isnt that interesting).
There's little doubt they are much stronger than humans though. There are enough online games that if this wasn't so it would show up.
https://en.m.wikipedia.org/wiki/Human–computer_chess_matches