You acknowledged them which was great, and then immediately turned around and started re-educating them

No, I provided the wider context. Please stop with the ridiculous strawmen.

Also:

pointing out that it's not universal or even the norm

That's called "denying someone else's experience."

No it isn't. That's like saying that it's denying the existence of octopi to say that there are other animal life in the oceans.

The original topic was whether or not the word "cis" is inherently a slur or an insult. I acknowledged their experience and then brought the discussion back on topic.

Again, it's about the worst way you can possibly comfort someone who is talking about when they were hurt.

They said themselves that they weren't overly bothered by it in the first place. Stop pretending that I'm callously retraumatising them by acknowledging their experience and then getting back on topic.

It doesn't matter if you think it's common: it happened to them

I fucking KNOW and I acknowledged it ffs!

The reason I brought it back on topic was that the anecdotal experience of one person, no matter how real and traumatic, doesn't override the fact that most resistance to the word is a bad faith effort meant to re-other trans people in order to more easily abuse and dismiss them with no negative repercussions for the abusers.

THAT'S what it's about and frankly, your continued insistence on missing the point is starting to smell suspicious..