Questionable Interview

So I was interviewing with a company (Cloudflare) and I thought it was going really well. I was interviewing for a new systems project they are building. However, during the interview, they made me go into very specific detail on how exactly I would implement things if I were to join their company (I'm talking all the way down to even specific algorithms to do stuff and system design). This went on to just over an hour and I had to go somewhere so we ended the conversation. On the same day, I get a rejection email. Did they just use the interview to gain some insight on how to develop this system? Or am I totally overreacting about this? I felt kind of used when I read the rejection email. When I asked for feedback, I never got a reply.