Plate armor isn't heavy at all. A full set of plate armor is like 20-25 kilos but it's spread out across your entire body so you barely feel it. It's a suit optimized for combat. It would be silly if it was heavy or awkward to use since you would die easily.
It was just a silly joke because they look like they just dropped to the ground out of energy just carrying around their armor doing nothing and they need a little rest.