There are no spoilers. It's a preview like for any game or movie just creatively written out and addressed from Gordon Freeman. It's weakly encrypted because if such a letter was written by Gordon to someone, it would have to be in order to keep certain details secure if the letter was intercepted.
Also consider the date. End of August is when most stuff starts popping up, which actually coincides with a fall release, with is not only great for the holidays, but the award shows in December. Most things are nominated based on the fact that it's the most recent awesome thing, involving its residual amazingness to propel the vote.
Look at the dates around the Halo games https://en.m.wikipedia.org/wiki/List_of_Halo_media
Making a good game isn't enough anymore, it takes an ocean of social media momentum and advertisement. They know they have something amazing, and shrouding it infamy will give it a bigger boost through the Internet grapevine for more effectively than any paid conventional ad source that their competitors may be doing.
The fact that it came from Laidlaw is the dead giveaway. He's a writer, and this isn't outside his realm of expertise. His departure from Valve was far too parachuted for him to be barred by a severance contract NDA. He is still perfectly within his right to do exactly what he did. His retirement from Valve isn't even two years old yet, and by the time he departed, his job could've easily been concluded with Ep3.
I think we'll see a release at some point this fall.