The only binary provided by sys-apps/opentmpfiles (0.1 and 9999) is /bin/tmpfiles. However, tmpfiles.eclass calls the binary "opentmpfiles (if systemd is not installed). It seems that the latter is a typo and should actually be "tmpfiles".
Giving this to William.
This is fixed in the following commit. https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=1fa75a8fbef3a4d6e1bcc858b9b4a984eeb14201