1. How did Photos tag this person?
2. Why are they showing up under People with just one image?
My only conclusion is that Apple somehow tagged this face using their own cloud AI, and that celebs get precedent.
If you download the photo from a media service, they may have tagged the face in the metadata (but I haven't personally witnessed Apple Photos extracting anything like that before).
You can view all embedded metadata by running `exiftool PATH-TO-IMAGE` (`brew install exiftool` first, if necessary).