I think you're right. But I also think your hypothetical interlocutor is too.

It's very easy to assume that the ideal way to play the game is to talk purely in terms of what your character does in the fiction...

It's easy, because 99% of the time, that's the more fun approach. You're totally right that this should not be approached dogmatically, but in my (substantial) experience, when the game is rolling that way and the non-MC players are in the moment and in the fiction, and the MC is doing most of the converting player-narrative into move-calls, it's just way more fun. That doesn't always work, because of mood, narrative, or internality of what's going on, and so there are other ways. But when it does, it's immersive.