My personal opinion is:
- The "60 degree angle" is certainly not canonical. I gave a couple of examples, and the one's above are good too.
- It's really not about the angle. The important material that Ueshiba was discussing had little to do with that.
In one sense, it's not really about angle. In another sense. it is about angle. Stand upright in a normal joe-schmoe posture with your feet just pointed neutrallyish forwardsish. Now squat down a bit by folding at the hip, and take note of how that felt. Then extend straight up and focus on extending the hip, take note of how that felt. Now turn your feet inwards so your toes are pointing at each other. Try to squat down again, then stand up again. Take note of how that felt. Now turn your feet out so yours toes are pointing straight out to the sides, the other extreme. Squat down, then stand up again. Take note of how that felt.
You might notice something like in the neutral orientation of the upper thigh with respect to the hip, force carries quite well, and the hip is quite strong. At either of the other extremes, the hip is just... weak, and easily loses connection with everything else, that connection with everything else being a very important thing to maintain. So if maintaining any stance that is to allow power to flow through, regardless of direction, that neutrality must be sought, regardless of the incidental starting position of the hip. If you take a kamae such that the hip socket is torqued, or rather, collapsed, to extremes, well, that is going to both impede external movement... and internal movement. The goal ain't to hold the hips in a position, the goal is to allow them to move actively in a powerful way.