I submitted this to understand why the author placed re0 in rdomain 1 which initially made no sense to me. I thought it would make it impossible to fetch the local address needed to setup the wireguard interface, or something.
When writing this comment as a question I realized why. The re0 shouldn’t be accessible for the rest of the system, and the wg0 interface becomes the default router anyways.
But hopefully this might be interesting to someone else. I had seen the term rdomain when reading documentation when setting up my home router, but now I know a bit more about it. :-)