i don't think hip flexors are really the most important muscle for getting faster and jumping higher...
squats make you jump higher because think about it, when you really try on a jump, do you not squat down before the jump? if you're not, you're supposed to.
i'm a basketball player and jumping has always been a big part of my game so i've always looked into ways to improve it.
Air Alert 2 is the program i started using for my vertical and the two most important exercises in it involve a form of squatting, and focuses on the same muscles.
the reason squats make you jump higher is they're a major body lift, they work more than just one muscle on your legs, and they are really done training in a natural jumping action, just in reverse. i mean yeah people talk about the calf muscles being important for jumping, and don't get me wrong they are. in athletic situations, you won't be squatting down as far and more of the power has to come from your calves. but if you're looking for just having a statisticaly high vertical leap, the squats will def. make you jump higher because of the muscles worked and the way they're worked.