[Google employee] Yes, you can use TPUs in Compute Engine and GKE, among other places, for whatever you'd like. I just checked and the v6 are available.
It's in the article: "When training the Llama-2-70B model, our tests demonstrate that Trillium achieves near-linear scaling from a 4-slice Trillium-256 chip pod to a 36-slice Trillium-256 chip pod at a 99% scaling efficiency."
I'm pretty sure they're doing fine-tune training, using Llama because it is a widely known and available sample. They used SDXL elsewhere for the same reason.
Llama 2 was released well over a year ago and was training between Meta and Microsoft.
Llama 2 end weights are public. The data used to train it, or even the process used to train it, are not. Google can't just train another Llama 2 from scratch.
They could train something similar, but it'd be super weird if they called it Llama 2. They could call it something like "Gemini", or if it's open weights, "Gemma".