I think you are mis-reading the documentation. Your first call is correct in retrieving the list of saved reports that were creating using the UI. However, as far as I know, it is not possible to actually execute any of those saved reports via the API.
Instead, to generate a cost report via the API, you need to use the /reporting/cost/run endpoint to create a new report by specifying the dimensions, metrics and filters. All of the examples in the documentation provide an idea of how to do that.
We use the cost/run endpoint within our ETL workflows to extract data from Cloudability on a daily basis that is then imported into our internal data lake which is then visualized via PowerBI and Tableau because we find that Apptio BI is extremely limited in its visualization capabilities.