It's well known that IVF intends to pummel Nothoi and then turn on Sint. Secession seems like a wonderfully strategic solution, in that case. I don't have a personal problem with "strategic secessions", but if this is the benchmark for an acceptable reason, it's time to revisit the matter in Arcaea.
You ignore the fact that it's basically one big war with two sides on it, with currently but two observers (Melhed and Fheuv'n). If Fronen fails, Enweil will fail. If Enweil fails, Fronen will fail. If Riombara fails, the North will fail, and if the North fails, Riombara will fail. Maybe not on the short-term, but on the long term it is without a doubt.
If Fheuv'n hadn't seceded, Rio's army would have been rofl-stomped when it showed up, and Enweil would have been able to more quickly press the attack on Rio. The quicker Enweil kills Rio, the sooner it can go move northwards, towards Sint, for example. And the sooner Enweil comes North, the bigger the trouble for these northern realms.
Politics in Enweil were stale. Over the months since the invasion ended, I had managed to recruit many new and old nobles and enlarge my duchy significantly with the TO of Pahk and Pequad and deal for Sandlakes. However, being border regions (and two of them newly-TOed), our stats were crap and control was a huge issue. So I had a bunch of nobles that could neither politically nor economically do much. And I knew that if I didn't do something to keep them interested, they'd eventually leave, and my ambitions would suffer a huge setback. Riombara's war declaration gave me a golden opportunity to break free, empowering my nobles and giving them something interesting politically to go on with.
As a smaller realm, we've got more leeway to experiment and do things differently. New realms are always extremely malleable after all.