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sufficiently advanced technology might make the construction of probes trivial, so that it has no great significance to its creatorsThe point is they bothered constructing probes.
My cat isn't constructing space probes. If he up and began doing so this evening, I would be able to conclude certain things about him.
> would be like asking America to account for a discarded Coke can
You're saying you can't conclude anything useful about American culture and civilisation from a discarded Coke can? (As well as the act of casually discarding it.)
> Aliens are completely unknowable, that's the thing most fiction trips up on
Aliens, yes. Aliens who make contact with us, no. The latter is a subset that requires certain attributes and heavily implies others.