Identity and authorization is a solve-for-first issue, with repercussions across a platform product implementation.
IDP is itself a capability that will need to be made available to platform users as well as incorporated into the platform. Effective productization of DI capabilities requires the abstraction of the platform user's identity from the underlying infrastructure IAM capabilities; as you are experiencing right now as you read this document on GitHub. Your identity integration with GitHub is not built around direct or SSO integration with their infrastructure providers IAM capability. Customer identity within GitHub is within an Abstraction layer.
If you are going to deliver a self-serve experience for internal consumers of a delivery infrastructure platform, how will you enable those internal teams to self-manage team creation and membership? When a team adds a team membership, how will that team member automatically have access to all of the team resources?
The Platform Starter Kit integrates Github and Github Teams information as AuthN and AuthZ, respectively. It is assumed that internal customers will already have been granted access to the company's GitHub organization and can self-manage team creation and membership. Using a Platform touchpoint (CLI, Developer Portal), internal customers can onboard their github team and automatically inherit this team-level authorization across all Platform resources.
PSK lab environments and working code examples make use of auth0.com for oauth2 workflows.