The OS isn't even ready yet and they're probably tweaking the brand new full fledged API that's probably full of bugs and issues and has to run on a completely new platform for Windows, ARM and also has to be battery efficient. You're underestimating the amount of work it takes port tens of millions of lines of code into a new platform and then to make sure things work and still have the ability to radically change things to get the last bit of performance out without having to worry about third party developers who have already invested significantly in the beta APIs.
Since they have already committed to allowing 3rd party browsers, I guess they will be coming forward with the information when it's ready. Till then the desktop browsers will continue to work for x86 based machines.