As of now man-db installs both /etc/cron.daily/man-db and /usr/lib/systemd/system/man-db.* files. The anacron files are not doing anything on my systemd gentoo. And the systemd files dont do anything for people that dont use systemd. Also the anacron useflag should make sure that a proper anacron is installed so it can be run.
Please see the Gentoo policy on this. https://projects.gentoo.org/qa/policy-guide/installed-files.html#pg0301