The underlying Config is the actual Typesafe Config object, and the play Configuration is a wrapper around that. The underlying getString method will throw an exception if the key isn't there, which is fine if you know for sure that key will exist (e.g. there's a value in reference.conf). If you are using Typesafe Config idiomatically, config keys should always have default values so you actually don't need the option.
I would still recommend using the Play Configuration API since it provides some nice helpers on top of Config. The API is likely to change in a future Play version to be more in line with the idiomatic usage though.