As stated in the documentation you shared, the Cloud CDN Custom Origin is intended to provide content hosted in another cloud (on-premises) and must be outside of the Google Cloud.
The Cloud CDN in use of HTTP(S) Load Balancer can be sourced for the following backends: Instance Groups, Zonal NEGs, Internet NEGs and Backend Buckets 1. Which the Internet NEGs relies on as a non-GCP service.
“Internet NEGs are also known as custom origins. Internet NEGs can be origins hosted within an on-premises infrastructure or origins provided by third-party providers.”
Let’s say that it is not compatible with App Engine or Cloud Run in GCP services.