On the Echo page: "If you are interested in being part of a limited-participation beta ahead of the SDK's public release, please provide your contact information."

I don't see any problem here: RIGHT NOW they're not ready to show anything PUBLICLY (and to be bound to an unstable API for example) but when everything will be completly cooked, they'll make it public

The NDA is just a way to ensure that the API wont become a de-facto liability and wont be made public by good souls... ;)