I have never seen anywhere that GitHub advertises using them as a CDN.
GitHub is for source control. That means a limited number of people pulling and submitting changes. That does not mean the general public using it as a CDN.
In fact I seem to remember seeing somewhere active discouragement of using it as a CDN.
That's fair, but they're really advertising a specific feature. That is, statically generated sites hosted based on a specific branch in a repository. Nowhere do they advertise themselves as a CDN in the way CocoaPods is using them now.
GitHub is for source control. That means a limited number of people pulling and submitting changes. That does not mean the general public using it as a CDN.
In fact I seem to remember seeing somewhere active discouragement of using it as a CDN.