I wrote too soon about Habermas and his use of Popper’s “third world”:
In what follows I shall no longer employ the Popperian terminology. My purpose in reviewing Jarvie’s action-theoretic translation of Popper’s three-world theory was only to prepare the way for the thesis that with the choice of a specific sociological concept of action we generally make specific “ontological” assumptions. (TOCA, 84-85)
So it is not clear at this point in the text what Habermas’s actual metaphysical commitments are. Later in this chapter, he provides a taxonomy of actions and is explicit about the ontological assumptions of each, and I would assume that Habermas is committed, at the very least, to the ontological assumptions of communicative action theories.
My guess is that the answer to the question of what it means for an expression to embody knowledge (DFN-EXP-EMBODY-KNOWLEDGE-?) lies within this more elaborate taxonomy.