Yea this is dumb. They use all three cloud providers. They started with AWS then started migrating some of their workloads to Azure in a bake off with GCP. They recently started using GCP for their workloads too. Apple even approached Azure to build a few data centers for them, but were turned down when Apple refused to foot the bill and only wanted to pay on a consumption basis.
Apple’s had huge issues with data centers in the past. There’s a great article in The Information talking about this.
A SaaS cloud service provider may use some other provider for IaaS or PaaS for that matter... Apple provides a service without necessarily owning the infrastructure service (could be counter intuitive but that’s a different discussion).