Does it? Nobody's arguing that trans women are female.
Their claim being that taking medication to suppress testosterone and boost estrogen, as well as having various cosmetic surgeries (castration, inversion of penis, bone/cartilage/soft tissue reshaping of facial features), gives these males a "female body".
Some of these males even claim to no longer be trans as a result of these surgical and pharmaceutical interventions, referring to themselves as "cis women".