So this is a lighter weight alternative to other Linux desktops?

Well, it can't run X or Wayland apps, so I wouldn't call it an alternative to those. An alternative to Haiku maybe.

why no X11?

Because everyone's going for Wayland, even though you still need XWayland to do anything useful on it because Wayland is comically incomplete.

well, I mean, why can't this ship with an X server? Or maybe it does and then I'm not sure why it is said that only Haiku apps are supported

It's not that it can't, if you can do something it doesn't mean you should. If we used X it'd be another linux distro isn't it? Part of the fun is to make your own UI feel.