Yes buy Van Phillips is not necessarily the general rule here. The program you link is designed to train certified prosthetists and orthotists, which are primarily people who custom fit prosthetics and orthotics to patients. Some may work in design, but that is not the typical job for those professionals (though I am sure their experience is valuable in the design process). The design of new prosthesis (not just custom fitting an existing design) is typically done primarily by engineers. They just aren’t the ones at the bedside fitting all the patients.
EDIT: Let me backpedal slightly. I shouldn’t say typically, but more accurately say “often.” It’s not surprising that a guy like Van Phillips designed the cheetah legs. That NU program includes courses like materials science and biomechanics so someone from there is probably equipped to design such things. That still does not mean there aren’t a sizable number of engineers involved, nor does it imply that the best way to get into the field is the way Phillips did.