I've heard of similar systems, and even relatively modest codebases (100K LOC) can easily accumulate many-CPU-hour build/test phases that rely on running across multiple build servers all running different OSes. None of which means that running tests locally isn't worthwhile, esp. if you rely on any form of TDD (personally I rarely actually write tests first, but certainly as I'm going to verify each new change. And I'm not waiting for our CI pipeline in that case, esp. because I often step-debug through the test execution to verify it's really doing what I intended)

It's a sort of an operating system for some specialized hardware. The code that runs AWS could be an example.