Just realized Duffield's tweet was June 29, and FYI, exactly one month later, Richard Dawkins released his interview of Helen Joyce in which Joyce made a similar claim: trans people can't claim "sexed" language like "mother." As you say here, the reasoning is more political than biological. Joyce made that explicit. She said (paraphrasing): If cis people give trans people their chosen gendered language, how ever will cis people justify taking away the trans person's gender at the moment the cis person wants to exclude them?