I don't think there is any reason to believe that github itself uses github actions for its ci/cd process. They must have had something in place before actions came on the scene, and even if they are migrating to actions it would likely be a long process.

I would just expect that if they want it to be a major feature, to dogfood it themselves. If it would be a long process to migrate, then how could they expect anyone else to migrate with the state of the feature?