This. I actually think this may have been a good move on their part. Showing off Halo 5 at E3 and then going, "That was awesome, wasn't it? You wanna play that shame, huh? Well too dang bad! It's not coming until late 2015! SO fudge YOU!"... that would have been very bad. It probably would have taken the wind out of a great many sails.
Doubt it. You've seen how the fans react to anything MS does. Showing a trailer and then "2015" at the end would hype them up. The rumors have been saying it anyway.