But it's not. When "it" is only defined circularly, it becomes impossible to lie about it.
Here's a made up example just to highlight the underlying logic issue. We take as a premise that some people are "helicals." And we define "helical" as "any person who asserts that they are a helical."
If I then say, "I am a helical," and I'm a person, I cannot be lying. It doesn't matter how I behave, what I believe, whether or not I claimed not to be a helical an hour ago, or whether I have a well-established reputation as a compulsive liar.
Changing the definition of "helical" to "any person who truthfully asserts that they are a helical" changes nothing, by the way, as any such assertion is already logically truthful.
Does the definition of transwoman include anything about any particular patterns of behavior, or any particular required beliefs, or any particular form of consistency? If not, it's not logically possible for a biologically male human to lie about being a transwoman.