It's self-deprecating humor. He knows it sounds narcissistic and expects it to be interpreted as ironic i.e. making the statement demonstrates that it isn't true. He's aware that he isn't charismatic and is letting you know that he knows it. It's like a weak person flexing their muscles to show you how strong they aren't.

(Although, there could be another level of irony, i.e. he is narcissistic, and worries you might think so, so he deploys irony to persuade you that he isn't by making a narcissistic-seeming comment in a self-undermining fashion—but I doubt it in this case).

In my experience, much British humor tends to follow a similar model (and Russian too perhaps), but I've observed that it's common for people who don't share that type of humor to misinterpret it.