The article also mentioned that they isolate by project_id. That implies one customer (assume a business) can isolate permissions more granulary.

Multi-database is more expensive generally but is a more brain dead guaranteed way to ensure the users are properly segregated, resilient across cloud/database/etc software releases that may regress something in a multi-tenant setup.

Multi-tenant you always run the risk of a software update, misconfiguration or operational error exposing existence of other users / their metadata / their data / their usage / etc. You also have a lot more of a challenge engineering for resource contention.