Psychological diagnosis (like those listed in the DSM) function to separate the socially acceptable from the unacceptable and do not even try to cut the world at its joints.

The difference is that post-modernists believe that something like this is true for all science and use this to justify this state of affairs in psychology, whereas LWers believe that this is not an acceptable state of affairs and should be fixed.

Edit: Also as MizedNuts pointed out, the diagnoses do try to cut reality at the joints, they just frequently fail due to social signaling interfering with seeking truth.