Of course, irony is difficult to define--hence the website: Is It Ironic?
From my perspective as a teacher of literature, the scene would be definitively ironic if the yakuza boss actually loved chocolate (upending the readers' expectations) or was allergic (upending the character's expectations).
So what is the humor here?
I asked the Translator.
Eugene: My theory is that the comedy arises out of a juxtaposition of roles and expectations, a clash of context-based social values that are valid in one setting but not another.
Probably the easiest form of this kind of comedy simply has a character not using the correct honorifics, like referring to a senpai by name only (yobisute). Here's a good explanation with a relevant example.
In The Way of the Househusband, the neighborhood housewives treat Tatsu like one of their own, so that's the role he assumes. But the yakuza treat him like one of their own too (as do the police). So you end up with collisions of wildly different expectations when the roles don't change to fit the setting.