So...what you're saying is that this person's gender identity and presentation doesn't match what you consider to be trans, or masculine enough to be trans, so you disbelieve their identity?
The way I read your description of this person, (if you consider the genderfluid identity as invalid, which I don't think you should) they could very well be a trans person in denial because of wildly swinging dysphoria, even if you discount what you consider to be not trans, ie gender variance, which can only be done if you are very strictly on the gender binary. A lot of people agree that gender is more of a spectrum than a binary, though I understand that it is not how you see it at all.
Personally, while absolutely considering my dysphoria as a sign of a medical problem, I am not comfortable policing other people's identities, just because they do not match up to my own experiences. I have no idea if people really can, with a physiological basis, change places on the gender spectrum, only that I know that I can't. Perhaps a very androgynous brain chemistry/build? I don't know. They could be intersex, for all you know.
What I do know is that deliberately misgendering someone seems cruel, especially if you already know what they identify as. Whether or not you agree with their identity is irrelevant, just as I am sure you've met people that do not agree with your own identity and have disregarded these. Respect peoples pronouns, even if you privately disagree.
Also, whether you find them attractive is irrelevant to whether you accept them or respect them.