This is just odd. They posted it to twitter a few times but it was also posted to reddit which is easier to repost here. It’s, er, different. Superficially the same but utterly different art style: modelled edges instead of blended as in RotMG, 2D wall models, more restricted colour pallete, modelled shadows. Most importantly it’s not using embiggened pixels but 1:1 art especially for the one enemy. And it has Japanese text.
What does it mean? Is this a new style to be added to the game? A new style for the whole game (so the previous tech demo gets ditched)? Or just an utter fake using artwork from another game?
My guess is the third as that would be typical of them. It looks too good to be by the same people as made their tech demo. It would explain the Japanese text too – ripped off from some obscure Japanese game. But even if not, adding a whole new art direction is wasting time, for a game that was originally supposed to be released last year.