Interesting. Apple is already heavily into Kubernetes and other methods of orchestration, so I don’t see these hires as a sign that direction is significantly changing, since their operations departments are also so large. Here’s a report, for example, about an Apple Engineer giving a talk on their heavy use of Kube and Mesos last year. Apple Plans to Run Most of Its ‘Compute Management’ on Kubernetes – The New Stack
I certainly would expect more services to follow Xcode Cloud, and wouldn’t be surprised if abstracted application servers for iOS and universal apps were offered at some point.
Trying to determine which iCloud features might be held back by under-engineered infrastructure is difficult to say.
“Apple … has for years been considered a bit of a backwater in the tech infrastructure community, far behind companies like AWS, Microsoft, Google, Facebook and Netflix.”
Apple definitely has the money to build enough data centers to handle whatever they have in mind. Perhaps with this new batch of hires it has finally acquired the knowledge to solve iCloud’s problems.
That is my hope and better file/folder sharing! 
I’d be more nervous than hopeful….