To answer your question, people get discounts just for negotiating, no PR required. I would guess the conversation went something like this: "___ is how much we pay to host Etsy on bare metal right now. Can you match that?" "Yes."

I'm oversimplifying - there are a bunch of other considerations, like guarantees about future prices and capacity availability and whether having more flexibility makes it worthwhile for Etsy to pay more, but fundamentally Etsy looked at their current costs and asked Google to make an offer. (I would assume they actually had all three major players bid, and Google won.)

Also to your point, the announcement specifically called out Google's AI and big data capabilities. Those are workloads that can involve a lot of bursting, so the cloud might be especially attractive. Imagine trying to implement BigQuery in bare metal - you can do it, but you have to have a lot of extra servers lying around just in case someone tries to run an analytics query. The same goes even more strongly for something like a model training workload. For bursty enough workloads, running in the cloud can be cheaper than bare metal even given the cloud provider's profit margin.

Depending on how much model training Etsy wants to do, something like TPUs might have also given Google a fundamental edge in costs over bare metal or other providers. I don't know enough to know exactly what factors went into Etsy's decision, but I think these are reasonable ideas.