Off the top of my head, managing 100 different database instances takes a lot more work from the business standpoint than managing 1 database with 100 users.

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

Yes it’s exactly this. There’s not a neat permission boundary when you have users, orgs, projects, environments. Let alone when you add RBAC too.