SteinitzLives: As someone who winds up on both sides of the Petroff (yet I still love chess ;-), I know that when black tries to make the Petroff not look much like one early on, bad things happen.
Rarely do I offer annotative posts, (if you can call this that) but I can't resist on this one, so I for indulgence and apologize in advance:
4 Nd3 has a surprisingly pretty record for white despite its ugly appearance which would fill Steinitz with that sort of "showy horror" he once wrote about.
6 . . . c6 is where Rapport, who plays interesting chess, gets too interesting against a World Ch. for his own good. Deviate from Petroff's early boring looking moves at your peril (more w/ black than white).
8 . . . . Bf5, likely not as good as the safer Petroff looking Nbd7 or the tension breaking Qxe2+.
9 ..... h5 and 10 . . . Na6, how many "bad looking" opening moves can black play against MC and not be busted about 10 or fewer moves later?
15 . . .Ne4 looks desperate more than it looks coffeehouse, but some would say they are one and the same.
After 21 Rxf5, black should respectfully resign as he has no hope for a promising or sustainable attack against any GM, IM or FM he might be playing against, not to mention that it's Carlsen.
Perhaps it's a case of not resigning until a better looking move number. GM Yasser once talked about a game where he was busted on about move 19 but played on to move 30 so it wouldn't look so bad in the chess press. Kudos to him for honesty.
I am no fan of recently bad Carlsen behavior, 2021 and forward, but Rapport should not have denied him what is clearly a legit miniature.