The problem is that you’re assuming the past gender of trans women categorically.

To a cis person, to break a harmful stereotype in a snappy way.

This is not how I would talk about it to you, or anyone else that is looking for a conversation on the topic.

I used it for the same reason we introduce high school students to electron orbits as if they were little planets. Despite not being technically correct, it's a useful generalisation for someone who lacks a nuanced understanding of the topic.

Most trans people, but not all, do not experience their gender as having changed, yet most cis people, assume that all trans people experience a change in their gender when they transition. That is the source of a lot of harmful assumptions about our lived experience.

The point of my comment was to challenge the cis person's understanding so they question it, not to try and explain nuance. I absolutely acknowledge that it isn't an accurate comment for all trans people, but I also believe that it wasn't the space for nuance, because info dumping the person I was replying to would have increased the chance of them skimming over it without re-thinking their assumptions.

Reading this stuff today only serves to drag those anxieties back up, because if we assume your claim is true then I either wasn’t a boy back then, or I’m not a trans woman now

I don't assume either of those.

Rather, I understand that the quippy one liner I used to make a cis person pause and evaluate their assumptions about gender isn't sufficient to cover experiences like yours.