Yum proxy does not currently support sqlite entries and relies on the xml files being present. (In fact we remove the _db entries from the repomd.xml on fetch).
This was deemed okay at the time of implementation as createrepo does not support creating "database-only" repositories, or at least it hasn't done since 2008. However it appears that the remote repositories for AWSLinux have removed the xml entries making the repositories incompatible with NXRM.
The attached repomd.xml files show the difference between an AWSLinux remote and a CentOS remote.