Meh... all those Riombarans ever do is talk. They talk so much that they decided not to do anything for the fourth invasion, and ended up basically sitting it out. Besides, everyone knows you can't trust Riombarans.
I think you're mixing up Riombarans with Melites. Riombara had to sit out most of the Fourth Invasion because they did more than just talk, and has been trying for the last 6 years to get wars going against Enweil.
