Even through the shape of the jaw where males apparently have a larger, protruding jaw, it’s still not a foolproof way of distinguishing gender. Could just be the genetics where the arowana has PLJ (protruding lower jaw).
That’s why arowanas are bred in a large pond stocked with many arowanas to increase the chances of breeding, although there’s little way to distinguish the gender. Apparently Qian Hu arowana breeding has figured out a way to distinguish Asian arowana gender 75% of the time, although not proven.