Somehow I doubt it. Getting such an email from a human is one thing, because humans actually feel gratitude. I don't think LLMs feel gratitude, so seeing them express gratitude is creepy and makes me questions the motives of the people running the experiment (though it does sound like an interesting experiment. I'm going to read more about it.)

Not a PR stunt. It's an experiment of letting models run wild and form their own mini-society. There really wasn't any human involved in sending this email, and nobody really has anything to gain from this.