That's not entirely the case - the application has to run in the environment, and for legacy apps (which most application virtualization is targeted at) Windows is a pretty decent bet.

On top of that the App-V/XenApp packaging suite are top notch and provide avenues for enterprise integrations. (I'm making a big leap assuming it's built on one of them, but it doesn't really make sense to build from scratch, as interoperability/portability would be a hassle)