A central rhetorical device here is treating what is taken as impossible as though it were doubtful. A proposition rejected in reality can still be set out in conditional form so the audience can be answered within its own assumptions.
The maneuver does not alter the truth of the matter. It makes argument and rebuttal possible without simply repeating a denial.
The distinction between a proposition’s actual status and its temporary rhetorical framing is essential to understanding the passage.