Currently, emerge --sync repo arguments only work for repos that have the auto-sync attribute enabled. The auto-sync attribute is more useful for emerge --sync with zero repo arguments. When specific repo arguments are given to emerge --sync, it make sense to to sync those repos regardless of the auto-sync attribute.
Patch posted for review: https://archives.gentoo.org/gentoo-portage-dev/message/2088d8bae9c00902d65b249c939a25ee https://github.com/gentoo/portage/pull/119 In the master branch now: https://gitweb.gentoo.org/proj/portage.git/commit/?id=75f0936b8866b143643abcd6c302cd72fc71eef3
Fixed in portage-2.3.5.