Lem would probably argue that we only think Solaris is intelligent because we are intelligent and then project that characteristic that belongs to us onto Solaris (in hopes that it will be true). In reality Lem is arguing that we will probably not recognize alien "intelligence" because it will have likely evolved in some ways different than us. It is an interesting philosophical argument he is making and what I had honestly never thought of until reading this book. But, I can see his point.