It is everything in skating, without it you are limited in your ability to dig into the ice on your edges, drive through on extension and balance your center of gravity over top of your blades in a fundamentally stronger position. Guys like Kaliyev with weaker ankle flexion tend to splay their legs which shifts their center of gravity back onto the heel of their outside leg. It limits a players ability to change direction or speed quickly which in most instances is more important than top speed. This can be improved, but it is one of those genetic things that you either have or don't have, and if you don't have it there is a ceiling as to how far you can improve. If you are interested Jack Han writes about this extensively in his newsletters.