Availability Group and Distributed Availability Licensing

I read a document here 

https://cloudblogs.microsoft.com/sqlserver/2019/10/30/new-high-availability-and-disaster-recovery-benefits-for-sql-server/

And I just need the clarification.

Fist, they are talking about SQL Server 2019, but what about 2017, does it follow the same rule?

We are planning to have an Availability group with one Primary and two secondary, one for HA and the second for DR.

If the cost of the primary is based on 8 cores and every secondary has 8 cores, the cost is now only 8 cores instead of 16 or 24, can that be confirmed?

Thanks