Very interesting work! It is an attempt to extract the interfaces that were in the minds of the implementors of the Smalltalk-80 system's collection classes, but which couldn't be expressed in the language itself, because it has no interface construct. That's what I meant by "Languages like Smalltalk (...) don't have interfaces at all."
Don't have manifest types and don't have manifest interfaces.
Someone has already referenced "Adding Dynamic Interfaces to Smalltalk" [0] and looking back there doesn't seem to be any kind of demonstration that use of interfaces makes software faster to develop or less error prone or... [1]
