Fully open creation of pads by non-registered persons: That is possible. However, since it may introduce a resource problem, the pads are deleted after three months after their last access. This is the default behaviour and that of cryptpad.fr. Hosted instances can tune this (deactivate, extend or reduce the expiration time). Maybeyou saw the warning of expiry and concluded the “not open”.
Sign-up on a cryptpad server is rather straightforward with very little known to the server (e.g. no email address to recover password). But using without sign-in or sign-up is definitely possible. E.g. one of our most popular pad is an iframe of a web page that is visited by dozens of new IP-addresses every day: they all can use the editing in an anonymous fashion. Since that page has been created by a registered user, it will not expire.