No, they are not really responsible. If you had brought it to them right away, then you would have shifted into gear and thrown the rod. The bottom line is they did not cause the problem. For them to be responsible, they have to have caused the problem. This sucks for sure, but think about if it was your business, you wouldn't take responsibility for people's repairs who brought you in a vehicle that was a ticking time bomb. Not trying to defend dealers or anything, because I think they are usually crooks anyway, but in this case, it really doesn't sound like they did anything wrong.
Now if they were revving excessively trying to make it force itself forward, then that is improper "troubleshooting" and they should be responsible for that, but that will be very difficult to prove after the fact.
Though it seems odd to me that it RAN fine, then the engine ended up failing. If there was a tranny problem, but no engine problem, could that cause a thrown rod? Like if the tranny was locked up and he forced the engine? Then you might have a case.