root can't normally log in via ssh. Unless the default configuration is changed.

In OpenSSH root cannot login.

In TinySSH, which also implements the ntru exchange, root is always allowed.

I don't know what the behavior is in Dropbear, but the point is that OpenSSH is not the only implementation.

TinySSH would also enable you to quiet the warning on RHEL 7 or other legacy platforms.

Fwiw some distros ask if you want root access enabled on install; I assume there's always some chance of it being enabled for install stuff and forgotten, or the user misreading and thinking it means any root access.