It's as open as humanly possible, is a better way of putting it. Indeed ARM CPUs aren't fully open. Also, depending on your particular compute module and options, other things aren't open, like GPU or Wifi driver binary blobs. But the non-open bits are listed when you configure the machine, if this is particularly important to you.

If you absolutely insist on 100% open, there's theKintex-7 FPGA compute module: you can do whatever you want with it and it'll be totally open. But the price is... fantastical. But hey, it exists if you have the money 🙂