I wouldn't say they abandoned Terraria but I wouldn't say they finished it either. They promised a lot of things that never happened.
As for Dexxter saying they stopped because they finished it...
There really is no other way to say "no more updates" without saying "no more updates." After losing my spriter to a potentially competing game, and having to take some time off for my own personal reasons I figured the most honest way would be to just get it out now then taking several months off and then come back to announce that I am working on another game.
Also, this isn't being made by the same devs. Tiy is the only person from the terraria team working on it. Thus the resemblance in looks.
This game is NOT the successor to Terraria. You shouldn't even compare the two until we get to know more about Starbound.