هَذِهِ الرِّسَالَةُ لَيْسَتْ لِلْقَاضِي.
This letter is not the judge's.
لَمْ تَثْبُتْ نِسْبَتُ هَذِهِ الرِّسَالَةِ إِلَى الْقَاضِي.
Attribution of this letter to the judge has not been established.
The second statement is more restrained. It reports lack of proof, while the first asserts falsity of the attribution. A conflicting seal may justify withholding attribution without establishing the stronger claim that the letter cannot be the judge's.
This is a genuine rhetorical distinction because appropriate wording should express a conclusion at the strength supported by the evidence. Overstatement harms both the reasoning and the suitability of the utterance to its context.