That’s not exactly true. They can cancel the game and it gets annulled instead of making a move, just like happens elsewhere. There doesn’t seem to be any rule against doing that that I can find. There’s another thread where I asked about this. I’ll look for it.
Edit: This one OGS Ladder Rules - #5 by yebellz
I guess it’s like resigning in that you move up above them, but I was thinking of resigning as more of playing the game for at least a few moves then conceding in some way for some reason as opposed to cancelling 
What I mean by not exactly true, is that they don’t have to accept, by way of playing. They “accept” in that the game is created but not in that they must play or lose rating. Just they get bumped a spot off the one they were in in the ladder.