Okay so I've read a LOT of analysis of that line, and done a fair amount Myself. There are two camps on it: One group says Robert is a repressed trans guy. The other group says Joanne is pretending to lean trans, in order to present herself as an insider and speak from a position of both authority and sympathy, so she can lie to people better.

Personally, I only found out about that interview during the research phase of a blog article I had already decided to write, about how Robert really hates women and views womanhood negatively, so maybe he's trans. https://medium.com/@viridiangrail/does-jk-rowling-want-to-be-a-man-568cb1aa4ca5