Hi RJ,
I've reviewed both yaml files and pushed some changes to hopefully improve their clarity.
To answer your question: that ARN refers to the user or role that has permission to assume the "ccf-athena" or "ccf" role (that has permission to query AWS APIs for the necessary usage data). For ThoughtWorks internal use, we have a role called "CCFService" that we've given permission to assume the "ccf-athena" role, but that is unlikely to be the case for you or others, so we've updated the templates to reflect that. The reason we didn't use ${AWS::AccountId} for this ARN is that the user/role that assumes "ccf-athena" may not exist in the same account where you upload and execute the cloud formation template.
Here is an example: let's say you have one AWS account "123456789", and you want the user "admin" to assume the "ccf-athena" role in the same account, then this ARN would be "arn:aws:iam::123456789:user/admin".
Hope that helps, and enables you to get things working. Let us know how you go and if you have any more issues.
Best,
Dan