I rather like it. No, she doesn't have to accept the offer. She can become governor. But she will have shown that she doesn't believe she could win a do-over. There's still more cards to play in the next weeks, too.
It's a good move that doesn't cost Dino anything yet.
I think they (and King county) are merely stringing this along and letting time go by. Every day that passes is now their ally (now that Sam Reed has shown everyone that he's simply the Democrats best lap-dog). They want to get her certified and sworn in as Governor. Then there is fait accompli also working against Rossi.