I don't think he's talking about the ability for an admin to sync content by site.
I think he's talking about giving a users (not admin) the permission to only sync one site and not have the permission to sync the other site.
For example, you have 3 sites setup up in Multisite, Site 1, Site 2 & Site 3.
John is a content editor on Site 1
Julie is a content editor on Site 1 & Site 2
John should only be able to edit content and sync it on Site 1, he shouldn't be able to edit content on Site 2 and especially should NOT be able to sync any content on Site 2.
Julie should be able to edit content and sync for both Site 1 & Site 2.
Neither should be able to do anything to Site 3.
Nothing in the documentation explains how to set permissions by site for syncing.
Right now if I restrict "Backend Access" by site you can't even get the Sync list to display. So in order to sync any site you have to access to all the sites and have permission to sync them, since access to the sync module appears to be global.