Whether it's proper use (whatever that means) of the term "native HTML5" or not, what Microsoft is implying is that their browser is the only one that runs HTML5 (specifically some of the graphics and video layers) directly on top of Windows Vista/7 graphical subsystems tied directly to hardware. I'm sure it employs technologies like WPF, DirectX, and so on.
The competitors (Mozilla, Opera, Google, and Apple) support hardware acceleration, but they do it their own way--almost like they "hacked together" support for true hardware acceleration. Firefox and Chrome's rendering of complex 3D scenes is still jerky and relatively slow *especially* compared to IE. I've also noticed that Firefox's live preview renderings (for parts of Aero) are absolutely awful. They might as well not even exist at all.
I'm not really an IE user, but I have to give kudos to Microsoft for the raw performance of IE 9 and 10. It really takes advantage of modern hardware. Other browser vendors should stop mocking and take some solid notes.