No, this is unrelated to Ubuntu's authentication model. This is in fact
the default setting provided by the upstream OpenSSH developers.
/usr/share/doc/openssh-server/README.Debian.gz explains.
** Visibility changed to: Public
** This bug is no longer flagged as a security issue
** Changed in: openssh (Ubuntu)
Status: Unconfirmed => Confirmed
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[openssh] Root login enabled by default (security issue)
https://launchpad.net/bugs/56143
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