This is addressed on the linked page.

Quote:

We're using xwayland-satellite rather than Xwayland directly because X11 is very cursed. xwayland-satellite takes on the bulk of the work dealing with the X11 peculiarities from us, giving niri normal Wayland windows to manage.

xwayland-satellite works well with most applications: Steam, games, Discord, even more exotic things like Ardour with wine Windows VST plugins. However, X11 apps that want to position windows or bars at specific screen coordinates won't behave correctly and will need a nested compositor to run. See sections below for how to do that.