Complex sentence: Since that time, letters purportedly sent by Saint-Mars, which earlier historians missed, indicate that Mattioli was only held at Pignerol and Sainte-Marguerite and was not at Exiles or the Bastille and therefore it is argued that he can be discounted.
Split into simpler sentences: Since that time, letters purportedly sent by Saint-Mars, which earlier historians evidently missed, indicate that Mattioli was only held at Pignerol and Sainte-Marguerite. He was never at Exiles or the Bastille and therefore it is argued that he can be discounted.