Earlier quoted context omitted.
Your quote would seem to support the idea you object to. "I'm no Henry Fonda" explicitly draws a comparison between yourself and Henry Fonda, and this kind of rhetorical device is often used to suggest things that the bounds of propriety forbid you from saying outright. Imagine two political rivals campaigning against each other, Mr. Lowbrow and Mr. Classy. Mr. Lowbrow releases campaign literature to the effect that…
In this case, the author is clearly comparing another juror ("Henry") to Henry Fonda, not himself. Which is definitely not comparing himself to Henry Fonda.
You can tell me how this means he's not comparing himself to Henry Fonda.