Hi Ho,
I connected with some of our Sonar team.
Thanks for bringing this to our attention, it looks like this link might be a little outdated - I will look to getting it updated.
In the meantime, you should not need to edit the credentials file anymore. The role_arn is used as the asset_id. All AWS credentials are entered through the Asset/Connections system whether it is using keypairs or AWS Assume Role ( like in this case ).
But it's even easier if after you create the credentials asset you use the Discovery feature. All these values are automatically populated as the RDS instances are discovered.
------------------------------
Sarah Lamont(csp)
Digital Community Manager
------------------------------