Why doesn't the fellow who says, "I'm no speechmaker," let it go at that instead of giving a demonstration?

Hubbard’s quip targets a familiar social performance: the person who begins by disclaiming eloquence (“I’m no speechmaker”) and then proceeds to deliver a full speech anyway. The humor lies in the mismatch between professed modesty and the ensuing “demonstration,” implying that the disclaimer is either false humility or a bid for lowered expectations. More broadly, the line satirizes public speaking conventions—prefatory self-deprecation, ritual modesty, and the way people use disclaimers to manage an audience’s judgment while still taking the floor. It’s a compact critique of insincerity and needless verbosity, typical of Hubbard’s plainspoken, small-town observational wit.