Gitea does not use Github because the software can't handle their load requirements, but because they started using GH before Gitea had all the features they needed. They are actively working on hosting Gitea on Gitea, with the last piece being an importer to move over all their GH data.
Blender seems to have been ok manually migrating some things and abandoning other things, since they weren't likely to ever get a Phab->Gitea importer anyways.