So did he gain an advantage? Hamilton was closing in using the DRS but was not in a position to attack at the chicane, so in that sense Rosberg did not gain an advantage of track position. He was ahead before and he was ahead after. Status quo.
Rosberg did set the fastest third sector time - a 30.810s - of the whole race on that lap. Overall, it was his second fastest lap of the Canadian GP.
That suggests there was a significant time gain, although as the line is so soon after the corner it's reasonable to give the driver longer to cede any advantage.
"It comes from running the brake balance further forward to protect the rear brakes and that just makes it all the more difficult all the time," explained Rosberg after the race.
"I didn't think anything of it. I went straight, didn't get an advantage - well I did initially and slowed down in Turn 1 and 2 as is the norm to do, so as long as I didn't gain an advantage, then it is fine. And thankfully that is how they judged it."
Rosberg did lose a little bit of time in the first sector, although you could certainly argue that as a result of the cut he was a little further up the road. At the start of lap 25, he was 0.564s clear, at the end of the contentious lap, that advantage was 1.184s then, after the next lap, 1.065s.
But Hamilton's 26th lap was also his slowest in that phase of the race, so that also has to be factored in. With such small margins, potentially a gain of a few tenths with factors such as Hamilton's pace also to be considered, it's not a straightforward decision to make.
And given that other drivers had escaped short-cuts in similar circumstances it would have been inconsistent suddenly to penalise Rosberg just because he was leading.
So the stewards' decision to take no action, save for a warning, was the right one. It was in keeping with the wording if the rules, the instructions given to drivers and the established interpretation of the regulation.