Yes, of course it is Imperative (Tzivuy) - in binyan qatal (=qal), and so is
הייתם. I think we're using qatal in a different meaning.
"This Binyan in Hebrew literature is called Pa'al ( פעל ), and in other
Semitic Research literature called Qatal ( קטל ). This Binyan conveys the
simplest possible meaning among the Binyanim. Due to this, this particular
Binyan has also been nicknamed Qal, which means "light" or "easy" in Hebrew."