To be fair, that's rather a problem of the applications not supporting Wayland. You can share within Xworld and Waylandworld, but not between them. I assume that's a non trivial problem and inherent to Wayland's design.
You can screenshare just fine through xdg-desktop-portal. It has nothing to do with whether the DE / compositor providing the implementation of xdp uses X or Wayland or something else, and it has nothing to do with whether the application doing the screenshare has an X window or a wayland window or any window at all.