Ant, do you have any evidence that the surviving Ruusanian bouncers were even caught in the blast? Laa wouldn't have survived without Rain's help, so I doubt any of the others would have survived either unless they could actually avoid the attack. And it's just a good feat for the kid if he survived it; it doesn't detract from the feat in any way.
The real crux of it is that the Jedi who were caught in the attack were utterly obliterated by it, which is why it's so impressive for Zannah, although she also has other feats as a kid, including levitating without practise, one-shotting fully trained Jedi, disintegrating limbs with a thought etc. So just imagine how powerful she would become with two decades of formal training, and even then she was only equal to Bane, who, keep in mind, had surpassed the power of his orbalisk suit on his own as of Dynasty of Evil. Even a single orbalisk made Bane more powerful than he had ever been, including when he was amped by Lehon, so just imagine how powerful an entire suit with potentially hundreds of orbalisks latched onto his skin and fueling his power would make him.