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 Mr. Classy is a pussy. Mr. Classy responds "a less courteous person might observe that my esteemed opponent washed out of seventh grade... but I won't. This race is about the issues."
The particular rhetoric Mr. Classy is using there doesn't mean it makes sense to defend him against charges that he called Mr. Lowbrow stupid. (Hey, he specifically said he wasn't doing 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.
Imagine two political rivals campaigning against each other, Mr. Lowbrow and Mr. Classy. Mr. Lowbrow releases campaign literature to the effect that Mr. Classy is a pussy. Mr. Classy responds "a less courteous person might observe that my esteemed opponent washed out of seventh grade... but I won't. This race is about the issues."
The particular rhetoric Mr. Classy is using there doesn't mean it makes sense to defend him against charges that he called Mr. Lowbrow stupid. (Hey, he specifically said he wasn't doing that!)