You're right, it doesn't address the file descriptor leak, only the root restriction (well, user namespaces address that).
But that isn't really an issue with chroot (or namespaces). It's (1) that CLOEXEC is opt-in, not opt-out, and (2) that you need this poll hack to enumerate open file descriptors.
Addressed, yeah, but I would not say solved for the general case of all the namespaces.
"Just put me in a (sand)box" is actually really tricky with namespaces, and depends on if you started off as root or not.
More on using namespaces to drop privs: https://blog.habets.se/2022/03/Dropping-privileges.html (another backburner project)
It's early morning so I may be wrong, but my testing seems to show that actually yes you can still fchdir() your way out of a file system namespace.