Since Obsidian seems to be a complete failure at acknowledging this issue, I've been using the Gamepass version and bought the Steam version, for science! I did submit a ticket for this problem and the response I got sounded like it was just read and responded to by AI and there was no follow-up when I replied trying to clarify all the things it got wrong about the initial ticket.

The Steam version works! You start up the Steam version and log in with your Microsoft account to enable Crossplay, and everything works fine.

My suspicion is that for certain accounts, probably random, the Gamepass/Xbox version of the game is defaulting crossplay to disabled, effectively disabling multiplayer. Or possibly the first time run, crossplay was disabled or broken, and it can never be changed again because the Xbox version of the game doesn't have an explicit Crossplay option that can be changed. This seems even more likely since I found in the config file on the PC that the Crossplay flag is false. I did try changing that to True, but I don't think that config file is actually used, or at least not all of it, because the game didn't respond to several other changes I made. The actual used config file might be one of those that's hidden behind the Microsoft game install folder locked wall, and then it's likely synced across devices, which would be why the same problem exists on an Xbox, PC, or cloud streaming. The actual config file being hidden somewhere would make sense too because it would be different for every Xbox profile on the system, and on PC the Xbox account isn't necessarily the Windows account that's logged on.

Not sure if I finally got through to them with the needed information, but after the latest update today, I'm able to see my Xbox friends list when looking to invite players in the Xbox versions. So everyone with the Xbox network issue in the Xbox/Gamepass PC version should update and try again, in case it wasn't just a fix for my username. I haven't had a chance to actually connect to a multiplayer game yet, but since I can see the friends list without errors, I expect connecting will likely work too.