Is preparing instant coffee a brew? Preparing instant coffee is basically pouring hot water on dry coarse powder made out of a coffee drink.

Is it still called brewing?

It seems like a language shortcut, as the brew happened before making the instant coffee powder.
I'd agree with you.  There isn't a brew happening because the solution was already created.  What you are performing is a rehydration.  However, "let me rehydrate some coffee" is a pretty awkward turn off phrase.