"So that remained their plea," meaning: that remained their statement; that is: "Woe to us! Indeed, we were wrongdoers," "until We made them mown down, silent," meaning: they had perished and become still.
﴿فَمَا زَالَت تِلْكَ دَعوَاهُم﴾ أَيْ: فَمَا زَالَ ذَلِكَ قَوْلُهُمْ؛ يَعْنِي: ﴿يَا وَيْلَنَا إِنَّا كُنَّا ظالمين﴾. ﴿حَتَّى جعلناهم حصيدا خامدين﴾ أَي: قد هَلَكُوا وَسَكنُوا.