 Such a structuralist methodology, or corresponding forms of methodological structuralism, tends to be tied to a general assumption about the subject matter of mathematics, namely: mathematics is the study structures.  But accepting that assumption does not, in itself, involve any further views about the nature of those structures, at least not in any detailed and philosophically loaded way. In contrast, all the forms of metaphysical structuralism considered earlier are meant to provide such views.  This is exactly how they go beyond methodological structuralism (often by building on it). 