It's the calf muscle that keeps the ankle restricted to a shorter ROM. Try standing on 1 leg with your your knee bent forward a little. Your calf is definitely being used. If you relax it you'll quickly drop (and possibly hurt your achilles so be careful). A better example might be doing a vertical leap. At the start while you're dipping the calf is working eccentrically.
I'm not recommending midfoot over ball of foot. I just think too much is made of staying on the ball of your foot. I think keeping your foot dorsiflexed makes a better cue for most people than staying off your heels. Your weight should always be mostly on the ball of your foot though even if the heel is on the ground. The problem comes when your weight lands on the heel which is followed by the foot extending and slapping the ground. If you're wearing regular running shoes the heel should probably be touching the ground. I think even the pose site says that you're over extending the ankle if your heel doesn't touch the ground in normal running shoes.