Tafsir of Al-Anfal 8:21

His saying, Glorified be He, "And do not be" is a confirmation of what preceded it. That is, do not be, by way of violating the command and the prohibition, like those who said, "We have heard," such as the disbelievers and the hypocrites who claim to hear, "while they do not hear"—meaning, a hearing from which they benefit, because they do not believe in what they have heard, nor do they understand it with the true understanding it deserves.

The sentence is in the position of a hal (circumstantial qualifier) pertaining to the pronoun in "they said." The negated hearing is a specific type of hearing, yet it is mentioned in absolute terms to indicate that they have been reduced to the status of one who does not hear at all, by making their hearing equivalent to non-existence.