He's saying that tanks became less effective during WW2, at first offense was overpowering defense thanks to tanks, airplanes and improved logistics. During WW2 defense started getting closer to offense, at least on land (air power improved greatly and it was the thing which facilitated all major offensives).
For example during the Battle of Kursk German tanks couldn't break through the Soviet defense because they didn't have air superiority. The Soviets (and almost everyone else) had learned by that point how to organize an effective anti-tank defense.
He's saying that tanks became less effective during WW2, at first offense was overpowering defense thanks to tanks, airplanes and improved logistics. During WW2 defense started getting closer to offense, at least on land (air power improved greatly and it was the thing which facilitated all major offensives).
For example during the Battle of Kursk German tanks couldn't break through the Soviet defense because they didn't have air superiority. The Soviets (and almost everyone else) had learned by that point how to organize an effective anti-tank defense.