So apparently they had a bit asking players to not share info about the game, but you could technically back out of it without agreeing so legally they can post whatever they want. It feels like a case of "this is legal to do but maybe kinda shitty and valve might be upset". Basically the agreement was informal and not enforceable and the verge just said fuck it. They did get banned afterwards, but I think that and not working with them in the future is all valve can do.
Edit: didn't even require agreement, so honestly it's kinda fair game. I was a bit hostile calling it shitty, I felt like it was a loophole or something but it's more Valve just saying "hey pls don't" and the verge replying "no thanks", and eating the game ban since that's all valve can really do.
This isn't some grand conspiracy it's a closed beta for a video game. It's pretty normal to have an NDA or embargo agreement to get access. It sounds like valve just goofed the implementation. So yeah it's totally legal for them to post it, valve just might avoid giving them early copies in the future.