If they're using the same strategy they were a year or two ago when I first started hearing about Google AI making progress in this space then I think it's closer to the second thing.

IIRC, they had a model that was built to predict weather 15 minutes into the future and were just recursively running that to get days worth of predictions. If that's still the case then I would assume they have predictions for every 15 minutes. Back when I first read about that they were saying the accuracy started to drop off around the 7-10 day mark though, so not sure if they improved on that or changed their strategy though.