I agree with you. While I have great regard for the uniforms of all our services, even in the military itself, the organizer of the program prescribes the dress/uniform for the occasion.
As you report, the cap and gown is a temporary, traditional robe worn over the clothes one chooses for the day. A proud Marine could wear his uniform, and for the graduation portion of that ceremony don the educational robe. Afterwards, he’s free to remove the robe.
As you suggest, as soon as one variation in dress is permitted, these schools are bombarded by those who wish to dress in the weirdest ways.
Good post.
Not under the gown he can't.