Because being open about the development would mean everyone here would discuss it, and either put pressure on Chen to add/change things that would delay the project, or bash him for not taking into account the remarks.
Everyone scream when there is vaporware, and at least for this one, you can't as he has not launch the project before being ready.

That doesn't mean that he is closed to comments, so he choose people he trust to give constuctive feedback, in a way he can control (unlike this forum, me included).

Not usual in this temple of open-source/hw, but seems like something a project that want to succeed could do.

I don't thing thanks mean anything particular on this forum, as not two people uses them the same way, but you can see it as a feedback that his posts are read carefully by people expecting this product, and any information he will be ready to give (like this "not ready yet" post, which is a lot more than some other projects we often talk about...).