I'm afraid that's an outdated use of the term, like it or not. Self-id and the relatively recent online movement (embraced by Stonewall, and this is what they've been teaching in their training to employers) mean that "trans" means a person is - literally - the gender they say they are. Not necessarily the opposite sex, because that relates to the body and not a gender identity.
And to say that trans= a person with gender dysphoria is to medicalise and stigmatise the experience of being trans (and, of course, is inherently transphobic).
It's very difficult to discuss when there are different and unknown meanings of words floating about.