I think as telescopes get better we just keep noticing bigger structures. Maybe this is just the biggest one we know right now.
I feel like it'd take some amazing statistics and millions of years of data to detail out structures larger than our observable universe.
True it would have to be physically grounded somehow. Maybe an obscure and uncharacteristically benevolent hyper rich fella sets up camp in the north pole.
I guess if we want Santa to exist historically it would take a religious level of interest and investment in ancient times. He would have to be reveared as a god and donated to. Then the organization can have the resources to get everyones Christmas list and send out presents on Christmas.