It seems that the link is fine. Scroll a little down and you can test the collaboration in Letters (built using CKE5) or with CKE5 document build (switch the tab).
I should note that I am on mobile. On mobile I don’t see a demo in the link from the OP whereas the demo in the link I posted above is available on mobile.
Yes, that's correct. The demo for mobiles is intentionally hidden because both examples on ckeditor.com are right-now desktop-like. We'd be working soon on a solution that would provide much better UX for mobiles, once we do it we'd show a dedicated demo for mobile devices.