This is not to be confused with the Avalonia UI framework which remains free and open source.
XPF is a new project by the same team that allows existing WPF apps to be cross-platform (so enterprises can take their existing Windows-only apps and run them on MacOS, Linux and maybe WebAssembly). It’s in early stages so I don’t know how well it works.
Good for them. That’s actually a great plan for monetization. It’s not going to gatekeep Avalonia features for most users, and it’s something that businesses will be willing to pay for because it saves them dev time