The notion was that a pattern of articles that tend to support the same group should raise our estimate of a bias in favor of that group.
All of them could be perfectly accurate, incidentally - consider a journalist who investigates 100 stories for every one he publishes and only publishes those that are favorable; while none of the articles would contain actual falsehoods, clearly this is a bias.
Whether that's the case here is a much deeper question that I can't answer from my extremely limited skimming of the available media and this thread.
Is the claim that they are actually doing very well, and he was incorrect in his article? Or that he is biased merely for writing an article based on negative facts?