If the rhel-x86_64-server-5 channel was sync'd after RHEL 5.10 release then yes it will automatically include 5.10 bits. Whenever we sync the RHEL channels it automatically downloads the latest available packages/errata's.

After 5.10 release if rhel-x86_64-server-5 was sync'd then upon creating a new clone channel it would contain 5.10 bits.

But just to clarify - after the clone channel is created, later if you sync rhel-x86_64-server-5 the new packages/errata's will not be available automatically in the previously cloned channels. New Packages/Errata need to be added/cloned with manual intervention. Those won't get added automatically into the previously cloned channels. If you need to add the latest available errata/packages to previously cloned channels then to achieve this you can either go to the Cloned Channel's -> Packages/Errata tab and Add/Clone the required packages/errata. Also there is another spacewalk-clone-by-date utility script to clone software channels and errata up to specific dates ensuring any added packages have their dependencies satisfied.