In “I saw a lion delivering a sermon,” the verb “delivering a sermon” points contextually toward a person, not an actual lion. The clue blocks the ordinary animal reading; it does not create a stable dictionary sense of “lion” meaning “any human orator.”
A contextual clue determines the intended sense of this utterance. Assignment concerns the relatively stable connection between expression and conventional meaning. Treating them as identical would turn each new metaphor into a newly established lexical definition.
Thus the domain of discourse and the prohibiting clue must be kept separate, particularly in theories where imaginative inclusion makes a metaphor appear close to literal reference.