When author of overlay make the package for SRC_URI like https://someproject.org/downloads/sources-ver.tar.gz" he can provide source distribution mirror for the case if someproject.org will go down or became discontinued. mirrorselect tool should allow to select one of specific servers for the overlay from the list of all servers in the overlay mirror. Now it doesn't: https://wiki.gentoo.org/wiki/Mirrorselect
see also https://bugs.gentoo.org/790500