I agree completely with your first paragraph, and disagree completely with your second.
"Futility" is subjective, and the whole purpose of the thought experiment is to point out that our predication of "futility" or really any other purely mental construct does not become automatically inherited by a mind we create. These imaginary arbitrarily powerful AIs would definitely be able to model a human being describing something as futile. Whether or not it persues that objective has nothing to do with it understanding what we do or don't want.
"Futility" is subjective, and the whole purpose of the thought experiment is to point out that our predication of "futility" or really any other purely mental construct does not become automatically inherited by a mind we create. These imaginary arbitrarily powerful AIs would definitely be able to model a human being describing something as futile. Whether or not it persues that objective has nothing to do with it understanding what we do or don't want.