Lol it reads like a uno reverse card against the Linux trial articles in previous years:

converting my Linux gaming PC to Windows just to play the same games seemed silly

As far as I can tell the article comes from real Windows experience though.

Is it truly free and open source?

Yes, both podman and podman-desktop are FOSS under Apache-2.0 license. And they prefer more open image hosts like quay.io by default (dockerhub is still available OOTB for it to be a drop-in replacement of docker)

Does it run only in Linux?

No, it runs on Linux, Windows and MacOS (through VMs on the latter two) and my experience is that it works better than docker on non-Linux machines.

The headscale integrated one is mostly enough, you can choose to include the tailscale official ones with their URL as well.