Seems like you're saying the same thing, just using "social contract" differently. I think they use social contract not to mean binding, but to highlight the fact that Hyrums Law must be taken in the social context of the project. In the case of large SW company, the social contract would be to not break services, even when folks are misusing an API. And for a popular open source project, it would mean not breaking a widely used behavior, even if it isn't specified or officially supported. Determining the social contract seems to be precisely what you describe as "not a social contract".