إِنَّ الْخَطَّ مُتَشَابِهٌ.
The handwriting is indeed similar.
Inna strengthens the assertion about handwriting resemblance. It may be fitting when that resemblance itself is disputed. The simple al-khaṭṭu mutashābihun communicates the same basic fact with less emphasis. Neither version, by its grammar alone, proves authorship.
Resemblance of handwriting and established attribution are distinct judgments. Treating the stronger wording of the former as conclusive proof of the latter is an error of inference, not a legitimate rhetorical benefit of emphasis.