Just a guess. i honestly think the game was originally to plan on releasing on May 5th but it received a delay a few weeks back. Probably was a communication error.Reasoning for this is that the trailer that they uploaded originally said Available Now, which was a mistake and it was replaced. As well, a Mouse pad was released today to celebrate H2A PC release.
Link to Mousepad: https://www.microsoft.com/en-us/p/halo-2-anniversary-mousepad/8w3whql533vp
The game was actually never going to release today - it was always May 12. Sorry folks got jebaited somewhat - I don't disagree that Twitter interaction was a little misleading (esp given that we did release HCEA out of the blue with little notice, so I don't blame folks for thinking that was the case again). As soon as I saw the tweets I knew what the outcome expectation would be (and I tried to course correct this AM). And that trailer wasn't actually meant to be published today - that trailer is for launch on 5/12. There was a brief issue where it went live on the Steam page, I believe it was rectified.