John Gill Commentary Deuteronomy 3:6

"And we utterly destroyed them, as we did unto Sihon king of Heshbon, utterly destroying every inhabited city, with the women and the little ones." — Deuteronomy 3:6 (ASV)

And we utterly destroyed them Not the cities, but the inhabitants of them:

as we did to Sihon king of Heshbon ; they did not destroy his cities, for they took them and dwelt in them; but the people that lived there, as follows here:

utterly destroying the men, women, and children, of every city ; see (Deuteronomy 2:34) .