What makes you think beta is gated on art assets? Could be a number of things affecting when they start it, like having the server backend to support it. Supporting a beta of people around the country/world is quite a bit different than hosting a few dozen on local server. Plus they may want the game in a certain state of quality before starting.
The things they reveal in these weeks is a unlikely to be tied directly to beta. Although the pace of the reveals has increased from one every other week to one per week. Doesn't seem to be as much content released with each either. Short blurb, a few pictures. Not like the webcasts or Q&A they used to do.
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