So a male person cant identify as a transwoman if said individual doesnt alter his appearance?
There are no "male person" in the human kind. Only person with male characteristics. Sex is more complicated than just female and male.
But yes, someone with male characteristic can completely identify as transwomen without changing his appearance. But chances are that they will try to change it anyway because of gender disphoria or heteronormativity in society.
Also, some some trans (gender fluid people for example) will not try to change their physical appearance but remain as they are and simply tweeking the surface level (closes, make up etc.)
They would be wrong about telling me its not ok to use words as they are defined.
Which is basically the same as saying that they would be wrong to tell you they they are factually systematically oppressed by the usage of said words.
Not by you necessarily, but by society in general.
You usage participate in said oppression. There no, they are not wrong to tell you that you are not using the correct usage. They are telling you that you are using an oppressive usage.
And yes.. AGAIN, it has EVERYTHING to do with the fact of delegitimazing their identity. It's an indirect consequence of your usage.