PhD offer and waiting for other options
You are getting an offer from an elite institution in the middle of a pandemic. The job market right now is absolutely awful. I can appreciate that you want only the absolute best for your future, but the way you sketch this situation hardly sounds like a dilemma at all.
Additionally, as soon as you manage to publish a paper or two on the project on which you are hired, it may very well happen that your supervisor rewards your performance by letting you seek out further research directions yourself. Then, you can always steer your trajectory more towards the intersection of AI and cosmology.