That is pretty much exactly what I thought, which is why I am confused.
When I first built my HM, I already owned a Pi. So, I installed OpenWRT, set up networking, and played around with it while I was waiting for the HM parts to arrive. When I got everything assembled and booted it up, everything seemed like it was working fine. I didn't have to flash anything. Maybe first boot flashes it for you when it detects the HM. Now that I think about it, that's probably what is happening.