And our associations?
Perhaps, our associations too, are mere brain waves. The interesting thing about them is that we shall try to understand them whenever they occur. (Just like our puzzlement over our dreams sets us to attempt at an interpretation). (I am, here, following up a previous argument.)
I think Freud could be defended like this as well: Freudian slips are not so much proof of some subconscious thought, but are like the paths we find in the woods suggesting that others have walked there. The paths do not tell who walked there or what they were talking about when they did, but it suggests we could walk there as well. In regard to our associations and Freudian slips—that pop up for no obvious reason—we shall attempt to find a meaning to them assuming that they stem from our own history of experiences, as they do. Indeed, they do, but not for semantic reasons. So the semantics of our associations (and Freudian slips and dreams) are added by us in the present.
I think this is why Freud would confront his patients with their dreams, slips and associations: to make them interpret them and, hence, provide them with a meaning.
Yet Freud also thought of psycho-analysis as a science, so perhaps he did think that slips and associations really carried a hidden, subconscious semantics. I think that would be a mistaken view, but I leave it to Freud scholars to assess just what he thought about this.