Just FYI, I think Hubbard knows this and wrote "A measurement is an observation that quantitatively reduces uncertainty" because he was trying to simplify and avoid clunky sentences. E.g. on p. 146 he writes:

It is even possible for an additional sample to sometimes increase the size of the [confidence] interval... before the next sample makes it narrower again. But, on average, the increasing sample size will decrease the size of the [confidence] interval.