The term "game manager" has been used for a long time and I think it came about as a polite way of describing a guy who does positive things on the field, but nothing spectacular or dynamic. It was just nicer than saying, "He's adequate, but limited."
Over time, people began to associate that term with mediocre play, quite simply because it was being used when referring to mediocre QBs who sometimes won – despite their lack of special skills. At this point, anyone who uses it for Russell is just a rival fan or a woefully uninformed mediot. RW's play is full of spectacular and dynamic; he's a game changer, obviously.
But his play is also smart and efficient. He can "manage a game" in the sense of controlling tempo, preventing mistakes, and handling field position. Those are the qualities that, in the past, a QB without a deep ball, say, or without scrambling ability, would have to rely on for wins. The term eventually became associated with a QB who could ONLY do those things. That's where some ignorant fans and mediots (used to?) try to slag off Russell and mis-apply the old term "game manager" as an insult.
To me, it's less an indictment on RW and more an indication that the speaker knows very little of which they speak. Put another way, when used in a negative sense, "game manager" is just name-calling masquerading as football analysis.