This blog post is announcing:
1. Using clang as a frontend.
2. Using C2 (MS compiler) as a backend.
What the LLVM community (IE not MS so far, but participation is of course, welcome)) has been building:
1. Using clang as a frontend
2. Using llvm as a backend
and even more specifically
1. Using MS Headers + MS Libraries
2. Using clang as a frontend
3. Using llvm as a backend.
(see blog.llvm.org/2014/07/clangllvm-on-windows-update.html for a 9 month old view)
Some of the mingw32 folks have been working on replacing #1 with non-ms stuff, but let's put that aside for the sake of this description )
Using clang as a frontend is where almost all of the ABI compatibility happens anyway (and where various developers have been focusing), so you end up with VC++ ABI compatible binaries whether you use C2 or not, if you/they use the pre-existing clang stuff.
Whether they are doing that (and then lowering llvm ir to c2 ir) , or if they've built an alternative IR generation with clang (IE generating c2 ir directly from clang), that they haven't released, is not clear.
If they've built something that generates C2 IR from clang, they would have had to write their own ABI compat, which would mean it may be different than the binaries clang would build.
(exception handling is also codegen'd by llvm, so exception handling could end up different, though of course, compatibility is the goal)