I couldn’t find any resource that says PostgreSQL has native horizontal scalability. Please provide a reference to this stated capability.
Let me google that for you.
It is stated on the main Azure page for the product: https://azure.microsoft.com/en-au/services/postgresql/#overview
Here are the docs for this feature: https://docs.microsoft.com/en-us/azure/postgresql/overview#azure-database-for-postgresql—hyperscale-citus