Today's draw was a repetition of position, which is distinct from a draw offer. Repetition is clearly specified in 3.7.1.b (3) of the link.
17.Qb3 is Carlsen's only move that is not clearly worse. Presumably, he would have followed the procedure outlined in 3.7.1.b (3) by stopping the clocks, consulting the Arbiter's scoresheet, and writing Qb3 on his scoresheet to claim the draw. At this point, had the Arbiter determined that Qb3 did not lead to a triple occurrance of position, Carlsen's move would have been played on the board and Anand would have gained three minutes on the clock.
I have not watched the video of the end of the game, so I'm uncertain that my scenario is precisely correct. It may be that the players agreed to a draw with no objection from the Arbiter.
We did NOT see a draw offer. We saw a draw claim. That's a critical difference. The rules for the Candidates contained the anti-draw rule, it wouldn't surprise me if there was such a rule here, but I haven't been bothered to look.