That's only for Metro apps provided directly from the store AFAIK (ie, apps compatible with Windows RT). The regular desktop apps can create a Metro interface (like IE10 - it's the same program, just with different interfaces for desktop and Metro) that allow them to use their own rendering engine in Metro.
I should have been more clear.... I am talking about metro browsers only. After using IE10 metro I really like it and actually prefer it to the desktop.
They might do it but it seems like a lot of work for chrome/ff to create separate versions of their browsers for desktop and metro and use different engines since winRT will be a consideration.