SpaceX Power Hardware Engineer interview questions

Got a cold mail from Spacex recruiter via Linkedin in 2015. I replied that I am not interested. Got another mail in 2016. I replied that I am interested to hear more. Had a brief phone call with the recruiter. He asked for my resume and I sent it. After few days he asked to schedule a phone call with the hiring manager. A week later I had the call. The hiring manager explained me about the company, about his team, the project and asked me about my background and interest. Overall good conversation. After few days the recruiter sent an email that the next step is take home project (See interview question). I agreed and he sent it over email asking me to send my answer within few days. I took a few hours the same night and sent my design including full documentation, derivations, formulas, assumptions, design choices part numbers and next steps. Two weeks later I get a reply: "They were looking for much more technical depth into the design." This reply really made me upset, not because they decided not to proceed, but because this is the least constructive and actionable feedback I could get. Why not enough technical depth? What were the gaps vs. their expectation? How the solution should look like in their view? I think after putting several hours into the design a minimal technical feedback, that takes 10-15 minutes, or at least a viable reference solution is a fair ask. I tried to follow up twice with the recruiter and the hiring manager but no reply.