As I explained in: Dirichlet boundary of DG/RT/N1curl element? - #2 by dokken
locate_dofs_topological works on dofs associated with the a set of entities (say facets in your case).
A DG element has all its dofs associated with the cell, not the facets or vertices, as they are not shared with neighbouring elements.
locate_dofs_geometrical tabulates the coordinates of the degrees of freedom (their location in physical space if they are defined as point evaluations) and check if they satisfies the conditional sent in.