The stoat taking down the rabbit can be likened to a lion taking down a zebra.
After reading a bit this is an isolated incident of a tame otter and is indeed a remarkable feat. But it is a very rare occurrence and definitely not par for the course as with stoat and rabbits.
How can you say its a rare occurence, when those otters are still pretty unknown in their behavior? They kill big caimans and anacondas. Believe me, a fish is no big deal for them. Arapaimas are not known for their intelligence. Once pushed into more shallow water, he is doomed.