Yes, Rosberg was indeed lucky to have the same 60 horsepower advantage over the rest of the field that his team-mate had.
It has always been my view that Rossi intended to retire, had told Yamaha so, then changed his mind. He actually said at one stage I think after the 2008 title that he wanted to beat Stoner two out of three before retiring. I also recently found by accident an old David Emmett piece from immediately after the 9th title win in 2009 which quoted Jarvis both humouring Jorge as the future of the marque and directly saying he didn't think he would be able to talk Valentino into riding past 2010.
I think he means he was lucky because everything that went wrong at Mercedes that year happened to Rosbergs much more talented teammate.