I always find the use of the words "close your hip angle" confusing, but I didn't learn to ride hunt seat. One can close their hip angle (make it more acute) By several different means. Well, two , basically. YOu can either bring your thigh bone up by moving your knee forward, or you can can't your whole upper body forward , from the seatbones, and this will close the hip angle.
I know you would not want to close the angle by moving your knee forward, but you can also make your thigh bone close the hip angle by raising the stirrup and bring your whole lower leg up higher, like closing an accordian.
But I think when they say "close your hip angle" they mean to can't the upper body more forward, from the seatbone. Right?
In dressage, one would probably be thinkg more of openning the hip angle, and again, this can be done by either moveing the whole lower leg back further under the rider, or by moving the whole upper body back , from the seat bone (not from the waist, as this would not change the hip angle at all).
Our bodies have so many joints, I just think it's amazing how we are so articulated!
In dressage you want to kind of think of having an "open" front of the body so that you have the ear, shoulder, hip and ankle alignment that is in a vertical line.