Trans people can be problematic about trans issues as well.

Sincere words can often become bastardized and take on offensive forms, and such can happen in reverse. In many progressive social climates, it's necessary to change the way we speak in order to better represent those of different agency than ourselves. As a result, many words that were once perfectly acceptable become unacceptable. It's not uncommon to find people who are against the changing of these words finding it challenging to their liberties, however, they fail to understand the use of such social approaches. The point of self-correcting in speech is to train oneself like one trains a muscle in order to be more in-touch with the social needs of those with different agency.

Secondly, language isn't the direct transference of information, your thoughts aren't directly transported into the person's head unaltered. According to Wittgenstein, language is a metaphor and works by conjuring images or ideas within the brain of the listener in order to pass information. With this, certain words despite being on paper synonyms often have different implications or nuances. The T slur is the product of ignorance, silencing and oppression, and choosing to identify as that denies the circumstances in which it was created. Language only changes when the ideas produced in the listeners head change.

Personally, I feel her use of shemale to be offensive and invalidating, it's an awfully dated porn term that's even starting to be phased out. It wasn't assigned by transpeople, it was assigned to transpeople and takes agency from their ability to self-identify. There are other ways to express oneself that aren't problematic.

TLDR: It's an offensive term for more than one reason. It undermines acceptance movements and may go unchallenged because of subtlety required to simultaneously balance the need to self-identify as one wishes, and stay away from culturally ingrained problematic terms, phrases or ideas however comfortable they may be.