CIAM is not nearly as popular of a term as it needs to be. When most developers build apps today, they still look at their cloud provider's IAM or Active Directory for inspiration in design of their customer-facing systems. I think this article is actually understating the complexity. Conway's Law rules all and sometimes your systems and users won't even necessarily be in your control. I urge folks to look into policy engines like OPA[0] and ReBAC systems like SpiceDB[1] rather than reinventing the wrong wheel.
[0]: https://www.openpolicyagent.org
[1]: https://github.com/authzed/spicedb