FWIW, I believe that's not a mistake, just a consistent use of (novel?) jargon. From context I infer that the paragraph is distinguishing between the _hard requirements_ of the traits (quote: "the language contract says they cannot become unsafe") and the behavior human programmers would naturally _expect_ of the traits ("the social contract").

And yes, looking at https://docs.rs/misfortunate/latest/misfortunate/index.html and Ctrl+F'ing for "social contract," I see that usage very consistently. The entire point of Misfortunate is that its types correctly implement the language contract, while violating the "social contract" in various surprising ways. For example, causing a hash collision on every operation is a perfectly cromulent implementation of Hash, but violates the "social contract" of Hash. For another example, look at LoadLetter — throwing an error on every operation is a valid implementation of Read, but still violates the "social contract" of what a human programmer would naturally expect from something readable.