Apple is using MS Visual Studio

I wouldn't be surprised if the standard development environment for low level ARM work is Windows, with reference device drivers, development and test tools, IDE utility plugins, etc to assume Windows and VS.

Beyond this role, Apple also develop versions of iTunes, the iCloud client and such on Windows. They also provide device drivers for Windows as part of Boot Camp and those will all require in-depth development and test skills and tooling on Windows.

Visual Studio has been available for OS X for several years, but I believe the only language it supports is C# with GTK. (ETA I guess there are some other languages, but not C++/C).

The job listing is for firmware dev, and there are mcu tools that are only available on Windows unfortunately -- note also that it says in the same sentence gnu toolchain experience is necessary.