PowerBI API: license clarification

I’m creating some website with integrate PowerBI. The integration is via PowerBI API based on my license PowerBI Pro.

The websites provide the security layer and each customer can see a different PowerBI report. Based on the Microsoft documentation and this port on PowerBI community (Solved: PowerBI REST API License/Azure - Help - Microsoft Power BI Community) we have to pay only the license for a PowerBI Pro and consuming the reports via PowerBI API is unlimited free.

I googled a bit and I found this other link 

https://community.powerbi.com/t5/Service/Power-BI-Embedded-REST-API-cost-architecture/td-p/290177

I don't understand what the real licence policy is. Could you clarify what the company has to pay?