> I could be super wrong but I'd expect a Java programmer to become useful on a C# project faster than I'd expect an AWS person to become useful building out other cloud infrastructure.
I think that is indeed wrong. The major clouds are all pretty similar in terms of the commodity IaaS services - VMs, disks, networks, load balancers etc., right up to things like managed Kubernetes clusters (AWS EKS, Google GKS, etc.) The basic correspondence between those various components is obvious, and the underlying infrastructure is the same - you still end up with servers with disks and TCP network access running whatever OS you chose. The situation gets even less different if you use something like Kubernetes, since your interaction with clusters largely relies on Kubernetes tooling instead of the cloud provider's.
Someone very familiar with AWS should easily be able to find corresponding services in e.g. Google Cloud - I know because I just went through that, having years of AWS experience as a software developer and architect, and now contracting with a company that uses Gcloud. There are differences, but prior knowledge of what to expect tends to make it easy to find what you need.
The only real exception is when you get to proprietary SaaS products, like AWS DynamoDB or Google BigQuery. But these are managed services which require virtually zero maintenance, so it's really the software developers who have to deal with those differences.
The problem with switching from something like Java to C# is that, even aside from the language syntax and semantics differences, which one can pick up fairly quickly, there's a huge ecosystem of libraries and tools that all changes, and the details matter much more - what collection of classes/functions/methods does a library have, what are their names, what do they do, which ones are better than which other ones, etc.
The time to get up to speed on all that is going to be greater no matter what, just because of the sheer volume of detail involved. Not that a Java programmer couldn't get up to basic speed on C# relatively quickly, but to reach expertise with it and not have to consult docs for many little things, i.e. to get efficient and effective at it, takes much longer. I've done that too - I spent nearly two years at a company that used C#, and can't consider myself an expert at all, unlike with Java.