Some of it might be actual-obfuscation if there are other people in the room, sure. But equally-intelligent equally-polite people are still expected to dance the dance even if they’re alone.
Your last paragraph gets at what I think is the main thing, which is basically just an attempt at kindness. You find a nicer, subtler way to phrase the truth in order to avoid shocking/triggering the other person. If both people involved were idealised Bayesian agents this would be unnecessary, but idealised Bayesian agents don’t have emotions, or at any rate they don’t have emotions about communication methods. Humans, on the other hand, often do; and it’s often not practical to try and train ourselves out of them completely; and even if it were, I don’t think it’s ultimately desirable. Idiosyncratic, arbitrary preferences are the salt of human nature; we shouldn’t be trying to smooth them out, even if they’re theoretically changeable to something more convenient. That way lies wireheading.
That might be a fault with my choice of example. (I am not infact in fact a master of etiquette.) But I’m sure examples can be supplied where “the polite thing to say” is a euphemism that you absolutely do expect the other person to understand. At a certain level of obviousness and ubiquity, they tend to shift into figures of speech. “Your loved one has passed on” instead of “you loved one is dead”, say.
And yes, that was a typo. Your way of expressing it might be considered an example of such unobtrusive politeness. My guess is that you said “I assume that’s just a slip” not because you have assigned noteworthy probability-mass to the hypothesis “astridain had a secretly brilliant reason for saying the opposite of what you’d expect and I just haven’t figured it out”, but because it’s nicer to fictitiously pretend to care about that possibility than to bluntly say “you made an error”. It reduces the extent to which I feel stupid in the moment; and it conveys a general outlook of your continuing to treat me as a worthy conversation partner; and that’s how I understand the note. I don’t come away with a false belief that you were genuinely worried about the possibility that there was a brilliant reason I’d reversed the pronouns and you couldn’t see it. You didn’t expect me to, and you didn’t expect anyone to. It’s just a graceful way of correcting someone.