ما أنا قلت هذا
It was not I who said this.
“ما أنا قلت هذا” is more specifically directed at denying that the act of saying is attributable to me. An event-first “ما قلت هذا” can negate the speaking more simply, without the same initial focus on the speaker's identity.
The form does not by itself logically prove that another person made the statement. Attribution to another speaker requires some independent contextual support. Denying agency of the speaker and affirming someone else's agency are distinct claims.
The rhetorical reading must keep this distinction visible, rather than treating implied contrast as an unconditional existential assertion.