Only if it's rather oddly configured. It can emulate the entire stack, but it doesn't have to as long as your guest and host architectures match.

Isn't that KVM? Why don't they say they use KVM if it's the case?

Because they use QEMU, which in turn uses KVM where appropriate, or some Microsoft thing on Windows, or an Intel equivalent on Intel-Macs. EDIT: and since this app is for now only for Mac and Windows if I read the page right, it's probably not using KVM obviously.