When Anthony claims he is not a good orator, it is an example of what

Antony uses sarcasm in his rhetoric by hiding behind humility.

Explanation:

Antony starts his speech by saying that he is not a good orator but lays out one of the most eloquent speeches that by its own virtue turns the people against the assailants whom they were supporting till then.

What Antony did here was multifaceted

-He lowered the expectation set for him - He made himself seem humble and by extension trust worthy - He made it seem like his speech was thus heart felt only and not using rhetoric which made it more effective.

This made the audience fall for the rhetoric of his speech that was his goal.