Wouldn't the benefit be for the employee whose spouse can direct anger not at the employee but their manager? The benefit would thus be employees being less likely to have to choose between their relation or their job.

If I was the husband, I'd rather prioritize being able to swiftly handle my wife's aggressions over brushing off such an intimate challenge for the sake of an employer (who instead gets to do the said challenge).

Anyhow - the purported direct benefit, according to Alex St. John, is for "the wife or GF" as the whole point is to make her happy (from the linked presentation: "If the wife or GF is unhappy the engineer is gone").

Hence, my question: how does it help the wife to directly interact with her husband's boss? Put differently, what is it about directly interacting with her husband's boss that makes her happier than directly interacting with her husband?

Only if the boss getting a bunch of angry texts has any impact. If things continue on the same way, then it's only postponing that choice.