They seem to be referring to "duck typing". The typing principle exemplified by the statement: If it looks like a duck, walks like a duck, and quacks like a duck, then it's a duck. Does it matter that it's a waddling man in a duck costume saying "quack"? Nope, still a duck. You don't need an explicit interface and an explicit declaration that you're implementing it, or to implement it fully, you just need to implement the operations relevant to the use of the object.
You have a procedure or something that you want to test and it takes, as a parameter, a logging service? You don't want to instantiate a full logging service and set up the database, because that's heavyweight for a test and irrelevant for the particular test? Fine, you throw together a quick and dirty logger that answers the method `log` and pass that in instead. No need to know what the precise interface has to be, or implement or stub out all its other capabilities. You know it has a `log` method because the procedure under test uses it, so that's what you give your quick and dirty mock logger. No more, no less.
You have a procedure or something that you want to test and it takes, as a parameter, a logging service? You don't want to instantiate a full logging service and set up the database, because that's heavyweight for a test and irrelevant for the particular test? Fine, you throw together a quick and dirty logger that answers the method `log` and pass that in instead. No need to know what the precise interface has to be, or implement or stub out all its other capabilities. You know it has a `log` method because the procedure under test uses it, so that's what you give your quick and dirty mock logger. No more, no less.