I always found this issue of "focus" a bit strange. Two billion can buy a lot of focus. For example, say there was a separate company smaller than Google that snap outsourced their infrastructure to, allowing them to focus, then they bought that company - it comes out to the same.
The technical capabilities is a different story.
You know that Snap can hire more than 3 people, right? They can have people working on the infrastructure at the same time that someone else works on "the product" they want to build. That's what happened at Google, and as a side effect, they now have a cloud platform they can rent out.
There are some benefits to using cloud services in some cases. It's rare that 100% cloud is a wise deployment move, especially for companies that operate at Snap-scale.
Also, by buying from Google, they hedge against lower than expected growth too. If growth doesn't meet expectations, while they're still obligated to spend the $2B, they can re-sell the services for likely close to cost, given they'll be getting a discount.