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.
Because hopefully those people will include, and (depending on population control) might indeed be overwhelmingly composed of, the current, pre-singularity population of Earth. I don’t think a majority of currently-alive humans would ever agree to destroy the Sun, and that includes being unwilling to self-modify into minds that would agree to destroy the Sun.
Raemon spoke upthread about how “no single culture that has survived 10,000 years”, but that was in a world with mortality.