It may not be possible to embed it, given Bibledit's layout mandates. But it is a challenging mystery!
The first picture (screenshot_1) Simply shows the Nextcloud window with top menu, including two external (embedded sites.) the first Icon, a cross, links to my bibledit-cloud instance. The last icon, a chat icon, links to my discussion forum instance.
The next screenshot (screenshot_2) shows the way that bibledit-cloud is rendering the embedded site. If you aren't already logged in, there is no way to access the menu because it is hidden by the top bar. Moving the menu down does not solve the issue. The screen continually reverts to placing the menus at the topmost line of the screen. I've tried to render it on various browsers, same result.
The final picture (screenshot_3) shows the discussion forum, also embedded, displaying correctly.