Nafs al-amr here denotes how things stand independently of an individual's mere opinion. Belief is a mental state; its correctness is tested by correspondence with the subject matter.
“This water is cold” is true if that water is in fact cold. An observer's confident but mistaken opinion cannot change its temperature.
Three questions must be kept apart: what is the actual situation, what does a person believe about it, and does that belief correspond to the situation? Human error does not entail that there is no determinate truth.