She's in her 50's. In my mom's case, I think its mostly just lived experience. If she was trans, she'd be less gay. She dresses in clothes from the men's section exclusively (including to her weddings), has shortish hair, is regularly mistaken as a guy (both in person and over the phone and I've never seen her correct anyone and she seems to be pretty happen about guy men thinking she's cute), is the only woman in her position at her jobs, is often treated more like a guy by friends and family while her fairly tomboyish wife is still treated as a woman, etc.
Her experiences of being a woman has basically just which sports teams she's allowed to play on because of the genitals she happened to have.
Granted, I have no clue what her opinion of her own body is. Personally, I always kept things like jealousy of girls or wishish I could just wake up in a different body to myself (until I started to seriously question my gender at 28yo). Mostly because I tried to avoid thinking about that kind of thing and pretend it didn't exist. Maybe she would still just consider herself a butch woman even if she had grown up in a different environment that was more accepting of trans people and had more understanding of non-binary/agender identities.
There are certainly some though that use it maliciously, the ‘you will never be a woman’ crowd.
My mom is a bit transphobic like that, but I think that belief came first and the transphobia came later partly as a result of that belief (and then she's adopted stupid talking points to further justify the transphobia). She only said that when asked about her own gender though: I've never seen her use it against trans people. Honestly, I haven't tried talking about gender with my mom (the closest thing I can think of is when she was complaining about singular they/them in bad faith, but I approached that as a discussion about the history of language, not gender... although my stepmom asked if I was in a woman in the middle of that conversation).