لَا نَنْسِبُ الرِّسَالَةَ إِلَى أَحَدٍ بِغَيْرِ بَيِّنَةٍ.
We do not attribute the letter to anyone without evidence.
Bi-ghayri bayyinatin prevents the listener from mistaking the researcher for someone who rejects all possible attributions. The actual position is conditional: attribution should follow evidence, not be ruled out even if evidence appears. That preventive role is an apt instance of iḥtirās.
Without the qualification, lā nansibu al-risālata ilā aḥadin could be heard as an unrestricted refusal to attribute the letter to anyone. The added phrase guards against that unintended reading rather than merely multiplying words.