Probably Microsoft was waiting for the E3 Media Briefing for the announcement, but someone must have turned on the switch early on the Games Store: as you can see from the two screenshots below,appeared in the store - though not accessible via direct search - and according to the pages it will take placewith a download size of 13 GB.

So it seems that Microsoft is preparing to let us try Creative Assembly's game during E3: get prepared if you're an Halo or RTS games fan!