The ebuild has an incorrect comment "mem_cache is replaced by cache_disk". See https://httpd.apache.org/docs/2.4/mod/mod_cache_socache.html - putting a "CacheSocache shmcb" line somewhere in httpd.conf should work, but nothing in the existing $APACHE2_MODULES list makes it work.
This is fixed in >=www-servers/apache-2.4.25