U r misinformed. What is definition of a traitor? Its someone who acts against the elected/prescribed authority for his personal gain.
Here, mujib was the elected president of Pakistan and had the support of the majority of the people of Pakistan.
But Pak regime especially its army didn't hand over the power to mujib since it would make it difficult for them to loot resources from East Pakistan.
They acted against the people of Pakistan, its elected president for their personal gain. Hence its quite evidence they were the real traitors. Pretty obvious, isn't it.