But my opinion that trans people are real isn't one of those opinions -- it doesn't have the same effect on cis people as the reverse does on me, because that opinion doesn't signify an existential threat.

I have seen white, straight, cis, men go into fight-or-flight mode when I attempted a basically mild critique of our company's lack of diversity in hiring. The first time it happened it was so unexpected that I didn't recognize the symptoms as communicating "I've just been massively triggered" and I totally failed to respond in a helpful manner.

So I think we can't actually know what will trigger any given person. I certainly wouldn't expect that expressing the opinion that trans people are real would have a triggering effect on cis people in general. It's weird, but I actually think that it's more likely to have a triggering effect on TERFs.

As far as what to do when someone in a privileged position has a triggered response to having their privilege challenged, obviously the onus is not on the person with less power to do caretaking when it happens — in my opinion, that's a very important piece of ally work that falls on other people with privilege to step up and take on.