tn The form of the suffix on this verbal form indicates that the verb is a preterite, not an imperfect. As such it simply states the action factually. Note as well the preterites with vav (ו) consecutive that follow in the verse.
snOlive oil from rock probably suggests olive trees growing on rocky ledges and yet doing so productively. See E. H. Merrill, Deuteronomy (NAC), 415; cf. TEV “their olive trees flourished in stony ground.”