Your rebuttal is that:
- The other higher polling and more well known politicians aren't "real" opposition politicians, they are just sock puppets.
- Navalny did well, once, in one city, in a race which Putin didn't participate in.
The first is your opinion only. You don't have data to support that, just your own political viewpoints. You being actually Russian doesn't make any difference; you can find people in the UK who think Tim Farron is the "real" leader of the UK opposition because he's actually trying to overturn brexit, but that isn't based on any hard data either.
The latter is irrelevant: I knew about that election and when the west says Navalny is "leader of the opposition" they don't mean against Sergey Sobyanin, obviously, as nobody in the west knows who that is. They mean against Putin. And regardless of what you may believe about the other politicians in Russia, "opposition leader" has a clear factual definition which he does not meet.
This political opinion is invariably phrased as if it were inarguably scientific fact in the New York Times and elsewhere. Your personal belief that it "feels" true doesn't change the facts: the media is lying.