I am not sure, but i dont really think tobirama meant that he created 'kage bunshin no jutsu', my reason is...minato claimed it was hiz jutsu, but several other shinobi had used that technique b4 him, and he wouldnt have claimed its his jutsu since he knew that, i think minato was referring to how he could teleport along with his shadow clones using hiraishin, thats why tobirama saw through the entire jutsu since he created hiraishin and also knew about kagebunshin.