This is just a question if credential manager uses encryption.
http://www.georgestarcher.com/splunk-stored-encrypted-credentials/
Yes, but anyone with a splunk login that has the capability in it's role:list_storage_passwords can dump them. Basically it is good enough to say the credentials are not stored in the clear in configuration files.
As you are Splunk staff, you can hit up the prodsec team if you need more explicit details.